Combustion exhaust gas treatment system

ABSTRACT

Five combustion exhaust gas treatment systems capable of removing dust and selenium (Se) in combustion exhaust gas and making harmless are proposed: (1) combustion exhaust gas is cooled to 350° C. or less, dust is separated, Se is transformed into insoluble compound, and Se is separated; (2) combustion exhaust gas is cooled to 350° C. or less, dust is separated, Se elution preventive agent is added, and dust is formed into scale; (3) dust in combustion exhaust gas is collected by dust collector, dust is formed into slurry by making use of part of circulation liquid in desulfurization apparatus, and tetravalent Se in dust slurry is transformed into insoluble compound, which is separated into solid and liquid; (4) dust is separated from combustion exhaust gas by dust collector, and it is heated to gasify Se, and is led into desulfurization apparatus, etc., and formed into slurry, and Se is made into insoluble compound; and (5) a treating agent for making tetravalent Se insoluble is added to part of circulation liquid in desulfurization apparatus, and it is sprayed into combustion exhaust gas to make Se insoluble.

FIELD OF THE INVENTION AND RELATED ARTS

[0001] The present invention relates to a combustion exhaust gastreatment system, more particularly to a combustion exhaust gastreatment system capable of easily removing dust and selenium (Se) incombustion exhaust gas containing dust and Se components, and makingharmless.

[0002] Hitherto, as the combustion exhaust gas treatment systeminstalled in a thermal power plant or the like, a combustion exhaust gastreatment system comprising a dust collector (usually an electrostaticprecipitator) for removing fly ash and other dust from the combustionexhaust gas, and a combustion exhaust gas desulfurization apparatus forabsorbing sulfurous acid in the combustion exhaust gas is generallyemployed.

[0003] Moreover, conventionally, as the combustion exhaust gas treatmentsystem installed in a thermal power plant or the like, a combustionexhaust gas treatment system comprising a dry dust collector (usually anelectrostatic precipitator) for removing fly ash and other dust from thecombustion exhaust gas, and a wet combustion exhaust gas desulfurizationapparatus for absorbing sulfurous gas in the combustion exhaust gas bycontacting with an absorbent slurry (for example, slurry containingcalcium compound) in the absorption column, and separating andrecovering gypsum as byproduct from the slurry in the absorption columnis generally employed.

[0004] Recently, handling of harmful impurities contained in thecombustion exhaust gas aside from sulfur oxides is posing a problem. Inparticular, in the combustion exhaust gas treatment system for coalfired boiler, the harmfulness of selenium (Se) contained at a maximumlevel of about 10 mg/kg in coal is a problem lately, and its harmlesstreatment is demanded.

[0005] Meanwhile, Se exists as tetravalent Se (main form: selenious acidSeO₃ ²⁻) which is easy to treat by making insoluble by treating agent,and hexavalent Se (main form: selenic acid SeO₄ ²⁻) which is hard totreat by making insoluble, and in particular the hexavalent Se is highin solubility (solubility at 20° C. is 95%) and is easy to elute.Besides, this Se has a toxicity similar to that of arsenic compound, anddisaster cases and emission regulations are known overseas, and it isnewly added to the list of regulated items also in Japan, and iscontrolled by the environmental standard (0.01 mg/liter), dischargestandard (0.1 mg/liter), and elution standard in landfill (0.3mg/liter).

[0006]FIG. 24 shows an example of prior art of combustion exhaust gastreatment system of this type (an example of combustion exhaust gastreatment system for coal fired boiler). In FIG. 24 and FIG. 25, thecombustion exhaust gas A emitted from a coal fired boiler 1 is sent intoa denitration apparatus 2 installed downstream of the boiler 1 to be ridof nitrogen oxides (NOx), and passes through an air heater 3 and a heatrecovery unit 4 of gas-gas heater (GGH), and is introduced into anelectrostatic precipitator (EP) 5, in which fly ash and dust areremoved. In succession, the combustion exhaust gas is guided into a wetcombustion exhaust gas desulfurization apparatus 7 by a fan 6, andsulfurous gas is removed in this desulfurization apparatus 7, and afterpassing through a reheater 8 of the gas-gas heater (GGH), it is led intoa stack 10 by a fan 9, and is released into the atmosphere through thestack 10 (FIG. 25).

[0007] On the other hand, fly ash and dust removed in the electrostaticprecipitator 5 are discharged from plural hoppers 5 a (dust recoveryunits) formed in the electrostatic precipitator 5, and are conveyed andcollected in batch by a conveyor 11. Thus collected dust B is eitherrecycled as cement material or the like, or discarded in an ash disposalyard (FIG. 24).

[0008] Herein, the desulfurization apparatus 7 comprises an absorptioncolumn, for example, in which combustion exhaust gas is introduced, andby contact of combustion exhaust gas with absorbent slurry (usuallyslurry containing calcium compound) in this absorption column, thesulfurous acid in the combustion exhaust gas is absorbed in wet process,and usually from the slurry in the absorption column, gypsum isseparated and collected as byproduct.

[0009] Incidentally, the heat recovery unit 4 of the gas-gas heater(GGH) may be also disposed immediately before the desulfurizationapparatus 7 as shown in FIG. 26.

[0010] In these combustion exhaust gas treatment systems, most of Se incoal (Se in combustion exhaust gas) is condensed at the downstream sideof the air heater 3 (that is, the position before introduction into theelectrostatic precipitator 5), and is removed by the electrostaticprecipitator 5 in a state being contained in the dust in combustionexhaust gas, and is directly mixed in the refuse in the ash disposalyard or in the cement material. To render Se harmless by conforming tothe elution standard, it requires a complicated and costlyaftertreatment of, for example, diluting the ash removed by theelectrostatic precipitator 5 in a huge volume of water.

OBJECTS AND SUMMARY OF THE INVENTIONS

[0011] In the light of the prior arts, it is an object of the inventionto present a combustion exhaust gas treatment system capable of easilyremoving and making harmless the Se components contained in combustionexhaust gas.

[0012] To achieve the object, five inventions are proposed.

[0013] A. First Invention:

[0014] (1) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing dust and Se components, comprisingmeans for cooling combustion exhaust gas to 350° C. or less, dustcollecting means for separating dust in the combustion exhaust gas, andSe treating means for transforming the existent form of Se in the dustinto an insoluble compound by adding water and treating agent to thedust separated by the dust collecting means.

[0015] (2) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing dust and Se components, comprisingmeans for cooling combustion exhaust gas to 350° C. or less, dustcollecting means for separating dust in the combustion exhaust gas, Setreating means for transforming the existent form of Se in the dust intoan insoluble compound by adding water and treating agent to the dustseparated by the dust collecting means, and solid-liquid separatingmeans for separating the slurry containing insoluble Se compounddischarged from the Se treating means into solid and liquid.

[0016] Preferred embodiments of the first invention includes thefollowing combustion exhaust gas treatment systems (3) to (6).

[0017] (3) A combustion exhaust gas treatment system of (1) or (2),wherein the dust collecting means is a dust collecting apparatusconstituted by forming a plurality of recovery units for separating andcollecting dust from the inlet side to the outlet side of the combustionexhaust gas, separating and collecting the dust collected from therecovery unit at the inlet side of the combustion exhaust gas and thedust collected at the outlet side separately, and introducing only thedust collected at the outlet side into the Se treating means.

[0018] (4) A combustion exhaust gas treatment system of (1) or (2),further comprising sorting means for sorting the dust separated by thedust collecting means into large particle size group and small particlesize group, wherein only the small particle size dust sorted by thesorting means is introduced into the Se treating means to make Seinsoluble.

[0019] (5) A combustion exhaust gas treatment system of (3) or (4),further comprising means for mixing the dust making Se insoluble by theSe treating means, and the remaining dust not making Se insoluble, sothat the moisture content may be 20% or less.

[0020] (6) A combustion exhaust gas treatment system of (5), furthercomprising means for forming the dust mixed by the mixing means intoscale.

[0021] In the combustion exhaust gas treatment system of the firstinvention, the method of making Se insoluble may be realized in variousforms, and representative forms include a method of forming slurry byadding water to the dust containing Se components separated andrecovered by the dust collecting means, adding treating agent to theslurry to make Se insoluble, and separating into solid and liquid, and amethod of spraying a solution of treating agent to the dust to immerseuniformly, and making Se insoluble.

[0022] In the combustion exhaust gas treatment system of the firstinvention, most of the Se in the combustion exhaust gas is condensed asbeing cooled by the cooling means, and is removed by the dust collectingmeans in a state being contained in the dust. In consequence, thetreating agent is added by the Se treating means to the dust separatedby the dust collecting means, and the existent form of the Se in dust istransformed into an insoluble compound. Accordingly, if the dust isdiscarded same as in the prior art, the Se elution standard issatisfied, and the Se is made harmless easily without requiringcomplicated aftertreatment.

[0023] By making Se insoluble only in the dust separated and collectedfrom the specific recovery unit at the outlet side of the combustionexhaust gas in the dust collecting means, the required amount oftreating agent and capacity of Se treating means can be reduced, and theSe is made harmless easily and at low cost.

[0024] That is, according to the study by the present inventors, it isknown that more Se is contained (deposited) in the smaller particle sizedust (ash) separated and collected from the specific recovery unit atthe outlet side, and the Se is made harmless on the whole only byapplying insoluble treatment on the dust of smaller particle size.

[0025] Moreover, by the same reason, by making Se insoluble only in thedust of smaller particle size sorted by the sorting means, the requiredamount of treating agent and capacity of Se treating means can bereduced, and the Se is made harmless more easily and economically.

[0026] Incidentally, in the case of the apparatus for making Seinsoluble only in part of the dust, by further comprising the mixingmeans, the dust making Se insoluble, and the remaining dust without Seinsoluble treatment are mixed so that the moisture content may be 20% orless, and therefore the moisture content in the dust can be easilylowered and handling is made easier in the disposal process of dust,without installing any huge equipment such as solid-liquid separatorthat requires wastewater (filtrate) treatment.

[0027] Moreover, by further comprising means for making the dust mixedby the mixing means into scale form, handling in dust disposal processis much easier.

[0028] The combustion exhaust gas treatment system of the firstinvention is intended to separate Se components contained in thecombustion exhaust gas from the combustion exhaust gas together withdust, and making harmless by further making it insoluble. The dust beingrid of dust including Se components is further led into the wetcombustion exhaust gas desulfurization apparatus, and SO is removed, andis made completely harmless by proper treatment by ordinary method, andis released into the atmosphere.

[0029] B. Second Invention:

[0030] According to the system by the first invention, although the Sein the flue can be easily made harmless, but in order to form slurry byadding water to the separated and collected dust, and to make Seinsoluble by adding and mixing insoluble treating agent to separatedinto solid and liquid, it requires wastewater treatment apparatus,solid-liquid separating apparatus, and other devices.

[0031] A second invention presents a combustion exhaust gas treatmentsystem capable of easily making harmless the Se contained in thecombustion exhaust gas.

[0032] (1) A combustion exhaust gas treatment system for removingharmful matter in combustion exhaust gas, comprising means for coolingcombustion exhaust gas to 350° C. or less, dust collecting means forseparating dust in the combustion exhaust gas, and mixing means foradding and mixing Se elution preventive agent and humidifying liquid orsolution of Se elution preventive agent to the dust separated by thedust collecting means.

[0033] (2) A combustion exhaust gas treatment system for removingharmful matter in combustion exhaust gas, comprising means for coolingcombustion exhaust gas to 350° C. or less, dust collecting means forseparating dust in the combustion exhaust gas, mixing means for addingand mixing Se elution preventive agent and humidifying liquid orsolution of Se elution preventive agent to the dust separated by thedust collecting means, and means for forming the dust mixed with the Seelution preventive agent and humidifying agent or solution of Se elutionpreventive agent by the mixing means into scale form.

[0034] Preferred embodiments of the second invention involve thecombustion exhaust gas treatment systems (3) and (4).

[0035] (3) A combustion exhaust gas treatment system of (1) or (2),wherein the dust collecting means is a dust collecting apparatusconstituted by forming a plurality of recovery units for separating andcollecting dust from the inlet side to the outlet side of the combustionexhaust gas, separating and collecting the dust collected from therecovery unit at the inlet side of the combustion exhaust gas and thedust collected at the outlet side separately, and introducing only thedust collected at the outlet side into the mixing means.

[0036] (4) A combustion exhaust gas treatment system of (1) or (2),further comprising sorting means for sorting the dust separated by thedust collecting means into large particle size group and small particlesize group, wherein only the small particle size dust sorted by thesorting means is introduced into the mixing means.

[0037] In the combustion exhaust gas treatment system of the firstinvention, most of the Se in the combustion exhaust gas is condensed asbeing cooled by the cooling means, and is removed by the dust collectingmeans in a state being contained in the dust. In consequence, the Seelution preventive agent and humidifying liquid or solution of Seelution preventive agent are added by the mixing means to the dustseparated by the dust collecting means, and the existent form of the Sein dust is transformed into an insoluble compound. Accordingly, if thedust is discarded same as in the prior art, the Se elution standard issatisfied, and the Se is made harmless easily without requiringcomplicated aftertreatment. Still more, because of the constitution ofadding humidifying liquid, Se elution preventive agent or solution of Seelution preventive agent, and mixing the dust and Se elution preventiveagent, or further forming the mixture into scale, as compared with theconstitution of forming the dust into slurry and mixing Se insolubletreating agent and the separating into solid and liquid and discarding,it does not require large equipment or apparatus for wastewater(filtrate) treating facility or solid-liquid separator, and handling indisposal of dust is much easier.

[0038] By making insoluble only in the dust separated and collected fromthe specific recovery unit at the outlet side of the combustion exhaustgas in the dust collecting means by introducing into the mixing means,the required amount of Se elution preventive agent and capacity ofmixing means and scale forming means can be reduced, and the Se is madeharmless easily and at low cost.

[0039] That is, according to the study by the present inventors, it isknown that more Se is contained (deposited) in the smaller particle sizedust (ash) separated and collected from the specific recovery unit atthe outlet side, and the Se is made harmless on the whole only byapplying insoluble treatment on the dust of smaller particle size.

[0040] Moreover, by the same reason, by making Se insoluble only in thedust of smaller particle size sorted by the sorting means, the requiredamount of treating agent and capacity of Se treating means can bereduced, and the Se is made harmless more easily and economically.

[0041] C. Third Invention:

[0042] (1) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a dry dust collector for removing dust in thecombustion exhaust gas, a desulfurization apparatus having adesulfurization column for circulating an absorbent slurry for absorbingand removing sulfurous acid, repulping means for dissolving the dustremoved by the dry dust collector in water to form into slurry, treatingagent feeding means for feeding a treating agent for making tetravalentSe insoluble into the dust slurry obtained in the repulping means, meansfor separating the dust slurry containing Se made insoluble into solidand liquid, and a piping system for introducing the separation liquiddischarged from the separating means into the absorbent slurry.

[0043] (2) A combustion exhaust gas treatment system of (1), whereinpart of the circulating liquid forming the slurry of the desulfurizationsystem is introduced into the repulping means, and is used as thesolvent in the repulping means.

[0044] (3) A combustion exhaust gas treatment system of (1) or (2),further comprising means for feeding filter additive to the dust slurry.

[0045] (4) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a dry dust collector for removing dust in thecombustion exhaust gas, a desulfurization apparatus having adesulfurization column for circulating an absorbent slurry for absorbingand removing sulfurous acid, means for introducing the dust removed bythe dry dust collector into the absorbent slurry, and treating agentfeeding means for feeding a treating agent for making tetravalent Seinsoluble into the absorbent slurry.

[0046] (5) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a desulfurization apparatus having adesulfurization column for circulating an absorbent slurry for absorbingand removing sulfurous acid in the combustion exhaust gas, and treatingagent feeding means for feeding a treating agent for making tetravalentSe insoluble into the absorbent slurry, wherein the combustion exhaustgas is introduced directly into the desulfurization column.

[0047] (6) A combustion exhaust gas treatment system of any one of (1)to (5), further comprising oxidation-reduction reaction control meansfor controlling the oxidation-reduction reaction in the desulfurizationapparatus, so that the hexavalent Se mixed in the slurry in thedesulfurization apparatus may be reduced by the sulfurous acid in theslurry to be tetravalent.

[0048] (7) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a dry dust collector for removing dust in thecombustion exhaust gas, a desulfurization apparatus having a cooling anddust collecting column disposed upstream of an absorption column, andpossessing a desulfurization column for circulating an absorbent slurryfor absorbing and removing sulfurous acid, means for feeding the dustremoved by the dry dust collector into the circulating slurry in thecooling and dust collecting column, and means for feeding a treatingagent for making tetravalent Se insoluble into the circulating slurry inthe cooling and dust collecting column.

[0049] (8) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a desulfurization apparatus having a cooling anddust collecting column disposed upstream of an absorption column, andpossessing a desulfurization column for circulating an absorbent slurryfor absorbing and removing sulfurous acid, and means for feeding atreating agent for making tetravalent Se insoluble into the circulatingslurry in the cooling and dust collecting column, wherein the combustionexhaust gas is introduced directly into the cooling and dust collectingcolumn.

[0050] (9) A combustion exhaust gas treatment system of any one of (1)to (8), further comprising a wastewater treatment apparatus for treatingwastewater from the desulfurization apparatus, and treating agentfeeding means for feeding a treating agent for making tetravalent Seinsoluble to the impurity slurry separated by this wastewater treatmentapparatus.

[0051] Any combustion exhaust gas treatment system of the secondinvention comprises means for feeding a treating agent for makinginsoluble the tetravalent Se dissolved in the slurry. The treating agentfor making tetravalent Se insoluble includes, for example, FeCl₃,Fe₂(SO₄)₃, chelating agent (e.g. Epolus MX-7 of Miyoshi Resin), and highmolecular heavy metal capturing agent (e.g. Epofloc L-1 of MiyoshiResin). The reaction of these treating agent for making insolubletetravalent Se is described later.

[0052] In the combustion exhaust gas treatment system of (1), most of Sein the combustion exhaust gas is removed by the dry dust collector in astate being included in the dust, and the dust contacts with water orliquid (solvent) in the repulsing means, and is dissolved into theliquid in the process of forming into slurry. As the slurry formingliquid (solvent), aside from the water supplied from outside the system,the treated water or slurry coming from each process in the system maybe used. Of the Se components dissolved in the dust slurry, at leasttetravalent Se is made insoluble by treating agent, and is discharged tothe solid phase side by the separating means (into the dust cake). Onthe other hand, the filtrate separated by the separating means isintroduced into the slurry in the desulfurization apparatus.Accordingly, hexavalent Se is also included in the combustion exhaustgas, and if it is not made insoluble by the treating agent and isdissolved in the filtrate, most of the hexavalent Se is introduced intothe slurry in the desulfurization apparatus, and reacts with thesulfurous acid absorbed from the combustion exhaust gas into the slurry,and is reduced to return to tetravalent Se.

[0053] Therefore, if hexavalent Se is contained in the combustionexhaust gas, in the stationary state, Se components dissolved in theslurry in the desulfurization apparatus or the circulation liquidcomposing this slurry are mainly tetravalent Se components, and in thewastewater treatment apparatus for treating discharge of circulatingliquid of the desulfurization apparatus, only by making this tetravalentSe insoluble, the Se elution standard may be easily satisfied, and thedesulfurization apparatus can be used as reduction reaction apparatusfor hexavalent Se, so that the constitution of the entire system may besimplified.

[0054] In the combustion exhaust gas treatment system of (2), since partof the circulating liquid for composing the slurry of thedesulfurization apparatus in the combustion exhaust gas treatment systemof (1) is used as solvent in the repulping means, as compared with theconstitution of feeding water separately, the quantity (circulation) andconsumption of water can be reduced.

[0055] In the combustion exhaust gas treatment system of (3), sincefilter additive is charged into the mixing means or separating means,the dehydrating performance in the separating means is enhanced, and asolid matter (dust cake) low in moisture content and easy to handle isobtained. As the filter additive, gypsum used in the desulfurizationprocess or the like may be used.

[0056] In the combustion exhaust gas treatment system of (4), most of Sein combustion exhaust gas is removed by the dry dust collector in astate being contained in the dust, and is directly led into the slurryin the desulfurization apparatus, repulped in the desulfurizationapparatus, and mixed with a treating agent for making insoluble.Accordingly, at least tetravalent Se components of the Se componentscontained in the combustion exhaust gas are mostly made insolubledirectly by the treating agent in the desulfurization apparatus, andmixed in the solid matter (gypsum, etc.) separated and formed from theslurry in the desulfurization apparatus and discharged, and theremaining Se components are also easily made insoluble and solidified bythe insoluble treating agent in the wastewater treatment apparatus fortreating discharge of circulating liquid in the desulfurizationapparatus. Besides, if hexavalent Se components are contained in thecombustion exhaust gas, most of hexavalent Se reacts with the sulfurousacid absorbed from the combustion exhaust gas in the slurry in thedesulfurization apparatus to be reduced to change to tetravalent Se,which is also made insoluble by the treating agent in thedesulfurization apparatus and is mixed into the solid matter (gypsum,etc.) separated and formed from the slurry in the desulfurizationapparatus and discharged, or easily made insoluble and solidified in thewastewater treatment apparatus.

[0057] Therefore, in this treatment system, too, the Se elution standardcan be easily satisfied, and moreover since the desulfurizationapparatus functions both as reducing reaction apparatus of hexavalent Seand as repulping means of dust, the constitution of the entire systemmay be further simplified.

[0058] In the combustion exhaust gas treatment system of (5), most of Sein combustion exhaust gas is directly introduced into thedesulfurization apparatus together with the combustion exhaust gas in astate being contained in dust, and is mixed into the absorbent slurryin, for example, the absorption column of the desulfurization apparatus,and is repulped and mixed with the treating agent. Accordingly, at leasttetravalent Se components of the Se components contained in thecombustion exhaust gas are mostly made insoluble directly by thetreating agent in the desulfurization apparatus, and mixed in the solidmatter (gypsum, etc.) separated and formed from the slurry in thedesulfurization apparatus and discharged, or easily made insoluble andsolidified by adding an insoluble treating agent in the wastewatertreatment apparatus for treating discharge of circulating liquid in thedesulfurization apparatus. Besides, if hexavalent Se components arecontained in the combustion exhaust gas, most of hexavalent Se reactswith the sulfurous acid absorbed from the combustion exhaust gas in theslurry in the desulfurization apparatus to be reduced to change totetravalent Se, which is also made insoluble by the treating agent inthe desulfurization apparatus and is mixed into the solid matter(gypsum, etc.) separated and formed from the slurry in thedesulfurization apparatus and discharged, or easily made insoluble andsolidified in the wastewater treatment apparatus.

[0059] Therefore, in this treatment system, too, the Se elution standardcan be easily satisfied, and moreover since the desulfurizationapparatus functions as the dust collector, reducing reaction apparatusof hexavalent Se, and repulping means of dust, the constitution of theentire system may be further simplified, as compared with theconstitution for installing the dust collector, repulping means andothers separately.

[0060] In the combustion exhaust gas treatment system of (6), theoxidation-reduction reaction control means controls theoxidation-reduction reaction in the desulfurization apparatus so thatthe hexavalent Se mixing in the slurry in the desulfurization apparatusmay be reduced almost completely by sulfurous acid in the slurry to betetravalent. Accordingly, if hexavalent SE is contained in thecombustion exhaust gas, this hexavalent Se can be almost completelychanged into tetravalent form in the desulfurization apparatus, so thatthe Se in the combustion exhaust gas may be made insoluble more easilyand completely.

[0061] In the combustion exhaust gas treatment system of (7), most of Sein combustion exhaust gas is removed by the dry dust collector in astate being contained in the dust, and is directly introduced into theliquid in the cooling and dust collecting column of the desulfurizationapparatus, and is repulped in the cooling and dust collecting column. Inthe dust slurry formed by feeding the dust containing Se into the liquidin the cooling and dust collecting column and repulsing, a treatingagent for making tetravalent Se insoluble is mixed. Accordingly, atleast tetravalent Se components of Se components contained in thecombustion exhaust gas are directly made insoluble by the treating agentin the desulfurization apparatus, and discharged into the solid-phaseside by separating means (into the dust cake), or are easily madeinsoluble by addition of insoluble treating agent in the subsequenttreatment of the separated water, so as to be discharged. Ig hexavalentSe components are contained in the Se components in the combustionexhaust gas, most of hexavalent Se components react with sulfurous acidabsorbed from the combustion exhaust gas into the liquid in the coolingand dust collecting column to changed to reduced tetravalent Secomponents, which are also made insoluble by the treating agent, anddischarged into the solid-phase side by separating means (into the dustcake), so as to be treated to be harmless.

[0062] Therefore, in this treatment system, too, the Se elution standardmay be easily satisfied. Moreover, since the desulfurization apparatusfunctions also as reducing reaction apparatus of hexavalent Se or asrepulping means of dust, the constitution of the entire system may befurther simplified. In this treatment system, since dust is not mixedinto the slurry in the absorption column of the desulfurizationapparatus, the performance of the desulfurization apparatus such asdesulfurization rate may be maintained high.

[0063] In the combustion exhaust gas treatment system of (8), most of Sein combustion exhaust gas is directly fed in the cooling and dustcollecting column of the desulfurization apparatus together with thecombustion exhaust gas in a state being contained in the dusts in thecombustion exhaust gas, and is repulped in this cooling and dustcollecting column. In the dust slurry repulped as the dust containing Seis fed into the liquid in the cooling and dust collecting column, atreating agent for making tetravalent Se insoluble is mixed.Accordingly, at least tetravalent Se components of Se componentscontained in the combustion exhaust gas are directly made insoluble bythe treating agent in the desulfurization apparatus, and discharged tothe solid-phase side (in the dust cake) by the separating means forseparating the dust slurry into solid and liquid, or is easily madeinsoluble by the addition of insoluble treating agent in the subsequenttreatment of separated water so as to be discarded. If hexavalent Secomponents are contained in the combustion exhaust gas, most of thesehexavalent Se components react with the sulfurous acid absorbed from thecombustion exhaust gas in the liquid in the cooling and dust collectingcolumn to transform to reduced tetravalent Se, which is also madeinsoluble by the treating agent, and is discharged to the solid-phaseside (in the dust cake) by the separating means, and is made harmless.

[0064] Therefore, in this treatment system, too, the Se elution standardcan be easily satisfied, and moreover since the desulfurizationapparatus functions as the dust collector, reducing reaction apparatusof hexavalent Se, and repulping means of dust, the constitution of theentire system may be further simplified, as compared with theconstitution for installing the dust collector, repulping means andothers separately. In addition, in this treatment system, dust is notmixed into the slurry in the absorption column of the desulfurizationapparatus, and the performance of the desulfurization apparatus such asdesulfurization rate may be maintained high.

[0065] In the combustion exhaust gas treatment system of (9), in a priorstage of treatment by solidifying the impurities in the wastewatertreating device in the desulfurization apparatus, a treating agent formaking tetravalent Se insoluble is added, and the Se eluting in thewastewater can be made insoluble to change into disposable form. In thecombustion exhaust gas treatment system of (1) to (8), Se in thecombustion exhaust gas can be almost completely made insoluble, but thissystem is effective in the case where Se is not to be made insoluble,for example, it is necessary to treat Se concentrated in thecirculation, being dissolved in the absorption liquid slurry due toreduction of hexavalent Se, mixing of Se into the gypsum to be collectedin the desulfurization process must be avoided. In the apparatus of (4)to (8), depending on the conditions of treatment, the insoluble treatingagent may be supplied only in the wastewater treating apparatus.

[0066] D. Fourth Invention:

[0067] (1) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing dust and Se components, comprising adust collector for removing dust from the combustion exhaust gas, andmeans for heating the dust removed by the dust collector to atemperature for gasification of Se in the dust.

[0068] (2) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a dust collector for removing dust from thecombustion exhaust gas, a desulfurization apparatus having an absorptioncolumn for circulating an absorbent slurry for absorbing and removingsulfurous acid, and means for heating the dust removed by the dustcollector to a temperature for gasification of Se in the dust, whereinthe gas generated by heating the dust by the heating means is fed intothe desulfurization apparatus together with the combustion exhaust gas,and Se is dissolved and captured in the slurry in the desulfurizationapparatus, and a treating agent for making the tetravalent Se insolubleis mixed in the treating process of slurry.

[0069] (3) A combustion exhaust gas treatment system of (2), furthercomprising oxidation-reduction reaction control means for controllingthe oxidation-reduction reaction in the desulfurization apparatus, sothat the hexavalent Se mixed in the slurry in the desulfurizationapparatus may be reduced by sulfurous acid in the slurry to betetravalent Se.

[0070] (4) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a dust collector for removing dust from thecombustion exhaust gas, a desulfurization apparatus having an absorptioncolumn for circulating an absorbent slurry for absorbing and removingsulfurous acid and a cooling and dust collecting column disposedupstream of the absorption column, and means for heating the dustremoved by the dust collector to a temperature for gasification of Se inthe dust, wherein the gas generated by heating the dust by the heatingmeans is fed into the desulfurization apparatus together with thecombustion exhaust gas, and Se is dissolved and captured in thecirculation liquid in the cooling and dust collecting column, and atreating agent for making the tetravalent Se insoluble is mixed in thetreating process of the circulation liquid.

[0071] (5) A combustion exhaust gas treatment system of any one of (1)to (4), wherein the dust collecting means comprises a plurality ofrecovery units for separating and collecting dust, from the inlet sideto the outlet side of the combustion exhaust gas, the dust collectedfrom the recovery unit at the inlet side of the combustion exhaust gasand the dust collected from the outlet side are individually separatedand collected, and only the dust separated and collected from therecovery unit at the outlet side is fed into the heating means.

[0072] (6) A combustion exhaust gas treatment system of any one of (1)to (4), further comprising sorting means for sorting the dust separatedby the dust collecting means into large particle size group and smallparticle size group, wherein only the small particle size dust sorted bythe sorting means is introduced into the heating means.

[0073] (7) A combustion exhaust gas treatment system of any one of (1)to (6), wherein the heating means is capable of heating the dust to anytemperature in a range of 100 to 1200° C.

[0074] In the combustion exhaust gas treatment system of (1), most of Sein combustion exhaust gas is removed by the dust collector in a state ofbeing contained in fly ash or dust, and is heated and gasified by theheating means. Accordingly, almost no Se is left over in the dust aftertreatment, and the Se elution standard is satisfied, and hence it can bedirectly recycled as cement material or discarded.

[0075] In the combustion exhaust gas treatment system of (2), most of Sein combustion exhaust gas is removed by the dust collector in a state ofbeing contained in fly ash or dust, and is heated and gasified by theheating means. The gasified Se is fed into the desulfurization apparatustogether with the combustion exhaust gas being rid of dust, and isdissolved and captured in the absorbent slurry. In the treating processof the absorbent slurry, it is mixed with a treating agent for makingtetravalent Se insoluble, and is made insoluble. That is, at leasttetravalent Se is directly made insoluble by the treating agent in thedesulfurization apparatus, and is discharged as being mixed in the solidmatter (gypsum, etc.) separated and formed from the slurry in thedesulfurization apparatus, or is made insoluble by the treating agent inthe wastewater treating apparatus for treating the discharge ofcirculation liquid in the desulfurization apparatus, and is easilysolidified.

[0076] Therefore, when the content of hexavalent Se is small in theabsorption liquid in the desulfurization apparatus, only by makingtetravalent Se insoluble, the Se elution standard can be satisfiedwithout releasing Se into the atmosphere. Moreover, in the constitutionwhere Se is separated from the dust by the heating means and isintroduced into the desulfurization apparatus, without feeding theentire dust into the desulfurization apparatus, the dust can be recycledeasily, and lowering of desulfurization performance in thedesulfurization apparatus can be avoided.

[0077] In the combustion exhaust gas treatment system of (3), theoxidation-reduction reaction control means controls theoxidation-reduction reaction of the slurry in the desulfurizationapparatus so that the hexavalent Se mixed in the slurry in thedesulfurization apparatus may be almost completely reduced by thesulfurous acid in the slurry to become tetravalent. Accordingly, thehexavalent Se can be changed to tetravalent almost completely in thedesulfurization apparatus, and the Se in combustion exhaust gas can beeasily and completely made insoluble.

[0078] In the combustion exhaust gas treatment system of (4), most of Sein combustion exhaust gas is removed by the dust collector in a state ofbeing contained in fly ash or dust, and is heated and gasified by theheating means. The gasified Se is fed into the cooling and dustcollecting column of the desulfurization apparatus together with thecombustion exhaust gas being rid of dust, and is dissolved and capturedin the circulation liquid. In the treating process of the circulationliquid, it is mixed with a treating agent for making tetravalent Seinsoluble, and is made insoluble. That is, at least tetravalent Se isdirectly discharged to the solid-phase side by solid-liquid separatingmeans or the like connected to the cooling and dust collecting column ofthe desulfurization apparatus, or is made insoluble by the treatingagent in the wastewater treating apparatus for treating the discharge ofcirculation liquid in the desulfurization apparatus, and is easilysolidified. Besides, most hexavalent Se reacts with sulfurous acidabsorbed from the combustion exhaust gas in the liquid in the coolingand dust collecting column and is reduced to be tetravalent Se, and ismade insoluble by the treating agent, and is discharged to thesolid-phase side by the separating means so as to be made harmless.

[0079] Therefore, in this system, too, the Se elution standard can besatisfied easily without releasing Se into the atmosphere, and thecooling and dust collecting column of the desulfurization apparatusfunctions also as hexavalent Se reduction reaction facility, so that theconstitution of the entire system may be simplified. Also in thissystem, Se or other dust rarely mixes into the slurry in the absorptioncolumn of the desulfurization apparatus, and the desulfurizationperformance in the desulfurization apparatus can be maintained high, andmoreover gypsum of high quality can be collected as byproduct.

[0080] In the combustion exhaust gas treatment system of (5), only thedust separated and collected from a specific recovery unit at the outletside of the combustion exhaust gas in the dust collecting means is fedinto the heating means, and Se is gasified and removed, and thereforethe required capacity of the heating means may be reduced. Moreover, inthe subsequent desulfurization apparatus and others, the required amountof the treating agent for making Se insoluble is also reduced, so thatSe is made harmless more easily and inexpensively.

[0081] That is, according to the study by the present inventors, it isknown that more Se is contained (deposited) in the smaller particle sizedust (ash) separated and collected from the specific recovery unit atthe outlet side, and the Se is made harmless on the whole only byheating dust of smaller particle size, and applying insoluble treatmenton gasified Se.

[0082] In the combustion exhaust gas treatment system of (6), Se isgasified and separated by feeding only the dust of small particle sizesorted by the sorting means into the heating means, and therefore therequired capacity of the heating means may be reduced. Moreover, in thesubsequent desulfurization apparatus and others, the required amount ofthe treating agent for making Se insoluble is also reduced, so that Seis made harmless more easily and inexpensively.

[0083] That is, according to the study by the present inventors, it isknown that more Se is contained (deposited) in the smaller particle sizedust (ash), and the Se is made harmless on the whole only by heatingdust of smaller particle size, and applying insoluble treatment ongasified Se.

[0084] In the combustion exhaust gas treatment system of (7), theheating temperature of dust by the heating means is 100 to 1200° C, andtherefore recondensation of gasified Se into dust is prevented, and Secan be easily removed from dust, and the Se elution standard of dust issatisfied.

[0085] E. Fifth Invention:

[0086] (1) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a dust collector for removing dust in thecombustion exhaust gas, a desulfurization apparatus having an absorptioncolumn for circulating an absorbent slurry for absorbing and removingsulfurous acid, means for mixing a treating agent for making at leasttetravalent Se insoluble, to a circulation liquid composing theabsorbent slurry extracted from the desulfurization apparatus, and meansfor spraying the circulation liquid mixed with the treating agent by themixing agent, into a combustion exhaust gas lead-in passage upstream ofthe dust collector.

[0087] (2) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a desulfurization apparatus having an absorptioncolumn for circulating an absorbent slurry for absorbing and removingsulfurous acid in the combustion exhaust gas, means for mixing atreating agent for making at least tetravalent Se insoluble, to acirculation liquid composing the absorbent slurry extracted from thedesulfurization apparatus, and means for spraying the circulation liquidmixed with the treating agent by the mixing agent, into a combustionexhaust gas lead-in passage upstream of the desulfurization apparatus,wherein the combustion exhaust gas is directly fed into thedesulfurization apparatus.

[0088] (3) A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a desulfurization apparatus having an absorptioncolumn for circulating an absorbent slurry for absorbing and removingsulfurous acid in the combustion exhaust gas, with a cooling and dustcollecting column disposed upstream of the absorption column, means forseparating the circulation slurry extracted from the cooling and dustcollecting column into solid and liquid, means for mixing a treatingagent for making at least tetravalent Se insoluble, to the separatedliquid discharged from the separating means, and means for spraying theseparated liquid mixed with the treating agent by the mixing agent, intoa combustion exhaust gas lead-in passage upstream of the desulfurizationapparatus, wherein the combustion exhaust gas is directly fed into thecooling and dust collecting column of the desulfurization apparatus.

[0089] (4) A combustion exhaust gas treatment system of any one of (1)to (3), further comprising oxidation-reduction reaction control meansfor controlling the oxidation-reduction reaction in the desulfurizationapparatus so that the hexavalent Se mixed in the slurry in thedesulfurization apparatus may be reduced by the sulfurous acid in theslurry to become tetravalent.

[0090] In the combustion exhaust gas treatment system of (1), most of Sein combustion exhaust gas is removed by the dust collector in a statebeing contained in the dust, but at least tetravalent Se thereof is,before being removed by the dust collector, made insoluble by reactingwith the treating agent for making at least tetravalent Se insoluble(hereinafter merely called treating agent) mixed in the circulationliquid of the desulfurization apparatus sprayed into the combustionexhaust gas lead-in passage by the spraying means. Accordingly, when theother Se content than tetravalent Se is less, the Se elution standardmay be satisfied if the dust after treatment may be directly recycled ordiscarded.

[0091] If hexavalent and other Se or other impurities mix into theslurry in the desulfurization apparatus, most of hexavalent Se reactswith sulfurous acid absorbed from the combustion exhaust gas in theslurry in the desulfurization apparatus, and is reduced to change totetravalent Se, and mainly tetravalent Se exists in the circulationliquid in the desulfurization apparatus. Consequently, this Se and otherimpurities are led into the mixing means as the circulation liquid isextracted, and mixed with the treating agent, and sprayed into thecombustion exhaust gas lead-in passage, and most Se is removed by thedust collector, together with dust, in an insoluble state. If Se andother impurities are slightly mixed into the desulfurization apparatuswithout being removed by the dust collector, the Se and other impuritiesare prevented from being accumulated excessively in the circulationliquid of the desulfurization apparatus by the functions of the mixingmeans and spraying means, so that the wastewater treating apparatus fortreating the wastewater of the desulfurization apparatus is not needed.

[0092] Moreover, by using the desulfurization apparatus having a coolingand dust collecting column upstream of the absorption column, fine dustparticles not captured by the electrostatic precipitator can be capturedin the cooling and dust collecting column, and hardly mix into theslurry of the absorption column, and therefore higher desulfurizationperformance is achieved, and the collected gypsum is higher in quality.

[0093] In the combustion exhaust gas treatment system of (2), most Se incombustion exhaust gas is directly fed into the desulfurizationapparatus, together with the combustion exhaust gas, in a state beingcontained in the dust, but at least tetravalent Se thereof is madeinsoluble by reacting with the treating agent mixed in the circulationliquid of the desulfurization apparatus sprayed into the combustionexhaust gas lead-in passage by the spraying means. Accordingly, at leasttetravalent Se contained in the combustion exhaust gas is directlydischarged as being mixed into the solid matter (gypsum, etc.) separatedand formed from the slurry in the desulfurization apparatus.

[0094] If hexavalent Se mixed into the desulfurization apparatus, mostof the hexavalent Se reacts with sulfurous acid absorbed in the slurryin the desulfurization apparatus, and is reduced to be tetravalent Se,and also finally reacts with the treating agent added by the mixingmeans to be insoluble, and is discharged as being mixed in the solidmatter (dust cake, etc.) separated and formed in the desulfurizationapparatus.

[0095] Therefore, in this system, too, the Se elution standard ban beeasily satisfied, and without requiring wastewater treating apparatus,the Se and others are prevented from being accumulated excessively inthe absorption liquid in the desulfurization apparatus.

[0096] In the combustion exhaust gas treatment system of (3), most Se incombustion exhaust gas is directly fed into the cooling and dustcollecting column of the desulfurization apparatus, together with thecombustion exhaust gas, in a state being contained in the dust, but atleast tetravalent Se thereof is made insoluble by reacting with thetreating agent mixed in the circulation liquid of the desulfurizationapparatus sprayed into the combustion exhaust gas lead-in passage by thespraying means. Accordingly, at least tetravalent Se contained in thecombustion exhaust gas is directly discharged as being mixed into thesolid matter (gypsum, etc.) separated and formed by the separating meansfor separating the circulation slurry in the cooling and dust collectingcolumn into solid and liquid.

[0097] If hexavalent Se mixed into the cooling and dust collectingcolumn of the desulfurization apparatus, most of the hexavalent Sereacts with sulfurous acid absorbed in the liquid in the cooling anddust collecting column, and is reduced to be tetravalent Se, and alsofinally reacts with the treating agent added by the mixing means to beinsoluble, and is discharged as being mixed in the solid matterseparated and formed by the separating means.

[0098] Therefore, in this system, too, the Se elution standard ban beeasily satisfied. Moreover, without requiring wastewater treatingapparatus, the Se and others are prevented from being accumulatedexcessively in the circulation liquid in the desulfurization apparatus.Still more, in this system, since dust does not mix into the slurry inthe absorption column of the desulfurization apparatus, thedesulfurization rate in the desulfurization apparatus, the purity ofgypsum, and other performances may be kept high.

[0099] In the combustion exhaust gas treatment system of (4), theoxidation-reduction reaction control means controls theoxidation-reduction reaction of the slurry in the desulfurizationapparatus so that the hexavalent Se mixed in the slurry in thedesulfurization apparatus may be almost completely reduced by thesulfurous acid in the slurry to become tetravalent. Accordingly, ifexisting in the combustion exhaust gas, the hexavalent Se can be changedto tetravalent almost completely in the desulfurization apparatus, andthe Se in combustion exhaust gas can be easily and completely treated.When the desulfurization apparatus comprises cooling and dust collectingcolumn, most of hexavalent Se is removed in the cooling and dustcollecting column, but where the cooling and dust collecting column isnot provided, the installation of this oxidation-reduction reactioncontrol means is particularly effective.

BRIEF DESCRIPTION OF THE DRAWINGS

[0100]FIG. 1 is a schematic explanatory diagram showing a constitutionof a combustion exhaust gas passing area of a combustion exhaust gastreatment system in embodiment 1 of the first invention.

[0101]FIG. 2 is a schematic explanatory diagram showing an essentialconstitution of the combustion exhaust gas treatment system in FIG. 1.

[0102]FIG. 3 is a schematic explanatory diagram showing a constitutionof a combustion exhaust gas treatment system in embodiment 2 of thefirst invention.

[0103]FIG. 4 is a schematic explanatory diagram showing a constitutionof a combustion exhaust gas treatment system in embodiment 3 of thefirst invention.

[0104]FIG. 5 is a schematic explanatory diagram showing a constitutionof a combustion exhaust gas treatment system in embodiment 4 of thefirst invention.

[0105]FIG. 6 is a schematic explanatory diagram showing a constitutionof a combustion exhaust gas treatment system in embodiment 5 of thefirst invention.

[0106]FIG. 7 is a diagram showing an essential constitution of acombustion exhaust gas treatment system in embodiment 1 of the secondinvention.

[0107]FIG. 8 is a diagram showing an essential constitution of acombustion exhaust gas treatment system in embodiment 2 of the secondinvention.

[0108]FIG. 9 is a schematic structural diagram of a combustion exhaustgas treatment system in embodiment 1 of the third invention.

[0109]FIG. 10 is a more specific structural diagram of the constitutionof the combustion exhaust gas treatment system in FIG. 1.

[0110]FIG. 11 is a schematic structural diagram of a combustion exhaustgas treatment system in embodiment 2 of the third invention.

[0111]FIG. 12 is a schematic structural diagram of a combustion exhaustgas treatment system in embodiment 3 of the third invention.

[0112]FIG. 13 is a schematic structural diagram of a combustion exhaustgas treatment system in embodiment 4 of the third invention.

[0113]FIG. 14 is a schematic structural diagram of a combustion exhaustgas treatment system in embodiment 5 of the third invention.

[0114]FIG. 15 is a schematic structural diagram of a combustion exhaustgas treatment system in embodiment 1 of the fourth invention.

[0115]FIG. 16 is a schematic structural diagram of a combustion exhaustgas treatment system in embodiment 2 of the fourth invention.

[0116]FIG. 17 is a schematic structural diagram of a combustion exhaustgas treatment system in embodiment 3 of the fourth invention.

[0117]FIG. 18 is a graph showing the relation between dust heatingtemperature and Se concentration in elution liquid in an elution test inembodiment 1 of the fourth invention.

[0118]FIG. 19 is a graph showing the relation between dust heatingtemperature and Se concentration in elution liquid in an elution test inembodiment 1 of the fourth invention.

[0119]FIG. 20 is a schematic structural diagram showing a constitutionof a combustion exhaust gas treatment system in embodiment 1 of thefifth invention.

[0120]FIG. 21 is a schematic structural diagram showing a constitutionof a combustion exhaust gas treatment system in embodiment 2 of thefifth invention.

[0121]FIG. 22 is a schematic structural diagram showing a constitutionof a combustion exhaust gas treatment system in embodiment 3 of thefifth invention.

[0122]FIG. 23 is a schematic structural diagram showing a constitutionof a combustion exhaust gas treatment system in embodiment 4 of thefifth invention.

[0123]FIG. 24 is a schematic structural diagram showing an example of aconventional combustion exhaust gas treatment system.

[0124]FIG. 25 is a schematic structural diagram showing other example ofa conventional combustion exhaust gas treatment system.

[0125]FIG. 26 is a schematic structural diagram showing a differentexample of a conventional combustion exhaust gas treatment system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0126] A: First Invention:

[0127] Referring now to the drawings, embodiments of the first inventionare described below.

[0128] (Embodiment 1)

[0129]FIG. 1 is a schematic explanatory diagram showing a constitutionof a combustion exhaust gas passing area in an example of a combustionexhaust gas treatment system of the first invention, and FIG. 2 is aschematic explanatory diagram showing an essential constitution of thecombustion exhaust gas treatment system. This combustion exhaust gastreatment system comprises, as shown in FIG. 1, a cooler 121 (coolingmeans) for cooling combustion exhaust gas A released from a coal firedboiler 120, an electrostatic precipitator 122 (dust collecting means,sorting means) disposed downstream of the cooler 121 for collecting dustsuch as fly ash in the combustion exhaust gas A and separating from thecombustion exhaust gas, and a fan 123 for feeding the combustion exhaustgas (being rid of dust) discharged from the electrostatic precipitator122 into a later process such as a wet combustion exhaust gasdesulfurization apparatus. At the dust discharge side of theelectrostatic precipitator 122, as shown in FIG. 2, a dust conveyor 124,an Se treating apparatus 125 (Se treating means) for making Seinsoluble, and a solid-liquid separator 126 for separating the dustslurry after Se insoluble treatment into solid and liquid are disposedsequentially.

[0130] This system may be incorporated, for example, in the conventionalcombustion exhaust gas treatment system shown in FIGS. 24, 25, 26, andconnected to the desulfurization apparatus for removing sulfurous acidfrom the combustion exhaust gas sent by the fan 123. Of course, theconventional air heater or gas h eater can be function as the cooler121.

[0131] The cooler 121 can set the outlet gas temperature, for example,in a range of 150 to 400° C., and the temperature may be set so that thecombustion exhaust gas may be cooled to a temperature sufficient forcondensing the Se in the combustion exhaust gas. More specifically, thetemperature is set so that the combustion exhaust gas may be cooled to350° C. or less, or preferably 310° C. or less. The lower limit of thecooling temperature is not particularly specified, but practically it isabout 90° C.

[0132] The electrostatic precipitator 122 has plural hoppers 131 to 134(recovery units) for separating and collecting dust, and these hoppers131 to 134 are formed sequentially from the inlet side (upstream side)to the outlet side (downstream side) of the combustion exhaust gas. Insuch constitution, dust of larger particle size is collected from theinlet side hopper, and dust of smaller particle size is collected fromthe outlet side hopper.

[0133] The conveyor 124 is design to convey dust B1 to B4 collected anddischarged from the hoppers 311 to 134 of the electrostatic precipitator122 into one place in batch (Se treating apparatus 125).

[0134] The Se treating apparatus 125 has a function of adding water orother liquid to the dust to form into slurry, and adding a treatingagent C thereto to mix, and, for example, the repulping tank for formingslurry and the mixing tank for adding and mixing the treating agent Cmay be separately disposed, or these functions may be realized by asingle tank.

[0135] As the treating agent C, a chemical reacting with SE to make itinsoluble is necessary, and if there is at least tetravalent Se (mainform: selenious acid SeO₃ ²⁻) among Se components to be removed, forexample, FeCl₃ or Fe₂(SO₄)₃ may be used.

[0136] If there is hexavalent Se (main form: selenic acid SeO₄ ²⁻) inthe combustion exhaust gas and it is needed to make it insoluble inorder to conform to the elution standard, as the treating agent, areducing agent for transforming hexavalent Se into tetravalent Se (e.g.Na₂SO₃) and the above chemical may be charged. In this case, apretreatment tank for reduction reaction of hexavalent Se intotetravalent Se may be provided aside from the mixing tank as theconstitution of the Se treating apparatus 125.

[0137] In this Se treating apparatus 125, if it is necessary to repulpthe dust, the circulation water of the desulfurization apparatus may beused as the solvent.

[0138] The charging amount of the treating agent C may be set slightlymore than the stoichiometric equivalent for making Se in the dustcompletely insoluble determined from the reaction mentioned below(reaction formulas 1, 2, or 3, 4).

[0139] In thus constituted combustion exhaust gas treatment system, dustremoval treatment and harmless treatment of Se contained much in thedust are performed as follows.

[0140] That is, the combustion exhaust gas A leaving the boiler 120 isfirst cooled by the cooler 121 to 350° C. or less, and therefore atleast at the downstream side of the cooler 121, the Se in the combustionexhaust gas is condensed and the majority deposits on the ash whichcomposes the dust. This dust is separated and collected by theelectrostatic precipitator 122, and all the separated and collected dustis fed into the Se treating apparatus 125 in batch by the conveyor 124.

[0141] In the Se treating apparatus 125, the tetravalent Se (main form:selenious acid SeO₃ ²⁻) contained in the slurry formed by repulping ofthe supplied dust (hereinafter called dust slurry) reacts with thetreating agent (FeCl₃ or Fe₂(SO₄)₃) as shown in the following reactionformulas 1, 2, or 3, 4, and is made insoluble in a form of iron selenite(Fe₂(SeO₃)₃).

[0142] When hexavalent Se (main form: selenic acid SeO₄ ²⁻) is present,the reducing agent as mentioned above is charged, and this hexavalent Sereacts with the reducing agent to be tetravalent Se, which similarlyreacts as shown in the following reaction formulas 1, 2, or 3, 4 to beinsoluble.

FeCl₃→Fe³⁺+3Cl⁻  (1)

2Fe³⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (2)

or

Fe₂(SO₄)₃→2Fe³⁺+3SO₄ ²⁻  (3)

2Fe³⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (4)

[0143] Accordingly, when the dust slurry making Se insoluble isseparated into solid and liquid in the solid-liquid separator 126, mostof Se is separated to the solid-phase side as iron selenite, and ismixed in insoluble form into the dust cake D discharged from thesolid-liquid separator 126. Therefore, if the dust cake D is directlydiscarded in the ash disposal yard, it conforms to the elution standard.Incidentally, since the filtrate E from the solid-liquid separator 126is extremely low in Se concentration, it can be easily treated aswastewater. Or it may be used as addition water for dust slurry, orreturned to the desulfurization apparatus as circulation water forabsorbent slurry of the desulfurization apparatus.

[0144] The result of combustion exhaust gas treatment experiment by thetest apparatus constructed as shown in FIGS. 1 and 2 is explained below.In the experiment, coal containing 3 mg/kg of Se was supplied into acombustion furnace at a rate of 25 kg/h, and combustion exhaust gas of200 m³ N/h discharged from the combustion furnace was cooled to 150° C.,and fed into an electrostatic precipitator. As a result, more than 99%of dust was captured by the electrostatic precipitator (Se capturingrate about 99.4%), and the amount of dust collected by the conveyor (thetotal collected from the hoppers) was 3.4 kg/h. As a result of analysisof the Se concentration in the dust (B1 to B4) collected by the conveyoraccording to the elution test and atomic absorption method by hydrogencompound generating method conforming to the ordinance No. 13 ofEnvironmental Agency of Japan, the concentration (Se componentscontained in the dust and eluting by elution test) was 0.33 mg/liter,which was over the landfill elution standard (0.3 mg/liter). However, in30 minutes after mixing by adding treating agent C (using FeCl₃) in theSe treating apparatus 125, by separating into solid and liquid by thesolid-liquid separator 126, the Se concentration was measured in thesolid-phase side (dust cake D) and liquid phase side (filtrate E), andthe results are shown in Table 1, which sufficiently satisfied theelution standard (0.3 mg/liter). That is, the eluting Se concentrationin the solid-phase side (Se components contained in the solid phase andeluting by the elution test) was 0.04 to 0.1 mg/liter, and the elutingSe concentration in the liquid-phase side was all 0.01 mg/liter or less.TABLE 1 Slurry FeCl₃ Eluting selenium Selenium concentration in concen-concen- concentration from solid liquid phase after tration trationphase after solid-liquid solid-liquid separation (wt. %) (wt. %)separation (mg/liter) (mg/liter) 25 0.1 0.10 All 0.01 or less 25 0.250.08 25 0.5 0.07 25 1.0 0.05 25 2.5 0.04 25 5.0 0.04 50 0.5 0.08 50 1.00.06 50 5.0 0.05

[0145] As described herein, according to the combustion exhaust gastreatment system of the embodiment, the Se in the combustion exhaust gasis removed together with the dust, without requiring the complicatedaftertreatment needed in the prior art, and is finally contained in thedust cake D in insoluble form, so as to be discarded directly. In thiscombustion exhaust gas treatment system, only by adding the Se treatingapparatus 125 and solid-liquid separator 126 to the conventionalcombustion exhaust gas treatment system shown in FIGS. 24 to 26, thecombustion exhaust gas treatment system incorporating the system of theinvention can be realized, and the Se in the combustion exhaust gas ismade harmless, and modification of the existing combustion exhaust gastreatment system is easy, and when newly installing this system, theconventional design or equipment may be used as it is.

[0146] (Embodiment 2)

[0147]FIG. 3 is a schematic explanatory diagram showing a constitutionof a second example of a combustion exhaust gas treatment system of thefirst invention. Same constituent elements as in embodiment 1 areidentified with same reference numerals, and their explanations areomitted. This combustion exhaust gas treatment system possesses, asshown in FIG. 3, a conveyor 141 for feeding only dust B3, B4 separatedand collected from specific hoppers 133, 134 at the outlet side of thecombustion exhaust gas, out of plural hoppers 131 to 134 in theelectrostatic precipitator 122, into the Se treating apparatus 125, anda conveyor 142 for conveying and treating the dust B1, B2 separated andcollected from specific hoppers 131, 132 at the inlet side of thecombustion exhaust gas, separately from the dust B3, B4, and ischaracterized by making Se insoluble only in part of the dust B3, B4,and discarding the remaining dust B1, B2. The electrostatic precipitator122 in this embodiment function as t he dust collecting means and thesorting means in the first invention.

[0148] In this case, only the dust B3, B4 separated and collected fromthe specific hoppers 133, 134 at the outlet side of the combustionexhaust gas undergoes Se insoluble treatment, and therefore the requiredamount of the treating agent C and the required capacity of the Setreating capacity 125 can be decreased, and the Se is made harmless moreeasily and inexpensively.

[0149] That is, according to the study by the present inventors, it isknown that more Se is contained (deposited) in the smaller particle sizedust (ash) separated and collected from the specific recovery unit atthe outlet side, and the Se is made harmless on the whole only byapplying insoluble treatment on the dust of smaller particle size,thereby contributing to reduction of facility cost and running cost.

[0150] The result of combustion exhaust gas treatment experiment by thetest apparatus in FIG. 3 is described below. In the experiment, coalcontaining 3 mg/kg of Se was supplied into a combustion furnace at 4rate of 25 kg/h, and the flue of 200 m³ N/h released from the combustionfurnace was cooled to 150° C., and fed into an electrostaticprecipitator. In this case, more than 99% of the dust was captured bythe electrostatic precipitator (capturing rate of Se about 99.4%), andthe amount of dust collected by the conveyors (the total collected fromthe hoppers) was 3.4 kg/h. The discharge amount of dusts (collected ash)conveyed and collected by the conveyors 141, 142, the mean particlesize, and eluting Se concentration (Se components contained in thecollected ash and eluting by the elution test) were as shown in Table 2.TABLE 2 Collected ash Collected ash Collected ash discharged fromdischarged from conveyor 142 conveyor 141 (combustion (combustionexhaust gas exhaust gas Item inlet side) outlet side) Discharge amountkg/h 2.27 1.14 Mean particle size of collected 12 5 ash μm Elutingselenium 0.20 0.49 concentration in collected ash mg/liter

[0151] More specifically, the discharge amount of the dust B3, B4separated and collected from the hoppers 133, 134 at the outlet side ofthe combustion exhaust gas was slight, 1.14 kg/h, but the eluting Seconcentration was 0.49 mg/liter, high above the standard. On the otherhand, the discharge amount of the dust B1, B2 separated and collectedfrom the hoppers 131, 132 at the inlet side of combustion exhaust gaswas large, 2.27 kg/h, but the eluting Se concentration was 0.20mg/liter, far below the standard. Accordingly, it is known that the dustB1, B2 separated and collected from the hoppers 131, 132 at the inletside of the combustion exhaust gas can be directly discarded. That is,Se insoluble treatment is not needed in the dust at the inlet side ofthe combustion exhaust gas which is about twice larger in the dischargeamount, and hence it is evident that the required amount of the treatingagent C and the required capacity of the Se treating apparatus 125 canbe substantially saved.

[0152] Incidentally, such difference in eluting Se concentration isregarded to be due to the particle size of dust (ash). That is, whengaseous Se (SeO₂) is condensed and deposits on the surface of the ashforming the dust, ash of smaller particle size is greater in thespecific surface area per unit weight, and hence more Se deposits. Onthe other hand, in the dust collector such as the electrostaticprecipitator mentioned above, coarse ash particles are likely to becaptured at the inlet side of the combustion exhaust gas, and fine ashparticles are likely to be captured at the outlet side of the combustionexhaust gas, and in other words there is a sorting function, and it ishence considered that the eluting Se concentration is high in the dustcaptured at the outlet side of the combustion exhaust gas.

[0153] The dust B3, B4 separated and collected from the hoppers 133, 134at the outlet side of the combustion exhaust gas were fed into the Setreating apparatus 125 by the conveyor 141, and the treating agent C(using FeCl₃) was added and mixed, and 30 minutes later, it wasseparated into solid and liquid in the solid-liquid separator 126, andthe Se concentration was measured in the solid-phase side (dust cake D)and liquid-phase side (filtrate E), and the measurements as shown inTable 3 were obtained, which sufficiently satisfied the elution standard(0.3 mg/liter). That is, the eluting Se concentration in the solid-phaseside (Se components contained in the solid phase and eluting by theelution test) was 0.05 to 0.09 mg/liter, and the eluting Seconcentration in the liquid-phase side was 0.01 mg/liter or less. Inembodiments 1 and 2, at the same slurry concentration and treating agent(FeCl₃) concentration, the consumption was the treating agent iscompared in Table 4, which shows that it is decreased to about {fraction(1/3)} in embodiment 2 as compared with embodiment 1. TABLE 3 SlurryFeCl₃ Eluting selenium Selenium concentration in concen- concen-concentration from solid liquid phase after tration tration phase aftersolid-liquid solid-liquid separation (wt. %) (wt. %) separation(mg/liter) (mg/liter) 25 0.5 0.08 All 0.01 or less 25 1.0 0.06 25 2.50.05 50 0.5 0.09 50 1.0 0.07 50 5.0 0.05

[0154] TABLE 4 Embodiment 1 Embodiment 2 Dust amount (kg/h) 3.4 1.14Slurry concentration (wt. %) 25 25 FeCl₃ concentration (wt. %) 0.5 0.5Consumption of treating agent (kg/h) 0.051 0.017

[0155] (Embodiment 3)

[0156]FIG. 4 is a schematic explanatory diagram showing a constructionof a third example of a combustion exhaust gas treatment system of thefirst invention. Same constituent elements as in embodiment 1 areidentified with same reference numerals and their explanations areomitted. This combustion exhaust gas treatment system, as shown in FIG.4, comprises a sorter 151 (sorting means) for classifying the dust B1 toB4 captured by the electrostatic precipitator 122 and conveyed in batchby the conveyor 124 into large particle size (coarse ash) B5 and smallparticle size (fine ash) B6, and only fine ash B6 sorted by the sorter151 captured by a fine particle capturing apparatus 152, and fed intothe Se treating apparatus 125 to make Se insoluble.

[0157] Meanwhile, in this case, the dust B1 to B4 conveyed in batch bythe conveyor 124 are conveyed by air F and led into the sorter 151. Thesorter 151 may be constituted by, for example, a cyclone, and it isconvenient when it is designed to adjust the degree of sorting. As thefine particle capturing apparatus 152, in this case, a bag filter isused.

[0158] In this case, only the fine ash B6 is subjected to Se insolubletreatment, and same as in embodiment 2, therefore, the required amountof treating agent C and required capacity of Se treating apparatus 125can be reduced, so that the Se may be made harmless more easily andinexpensively.

[0159] In this combustion exhaust gas treatment system, only by addingthe Se treating apparatus 125 and sorter 151 to the conventionalcombustion exhaust gas treatment system shown in FIGS. 24 to 26, thecombustion exhaust gas treatment system incorporating the system of theinvention can be realized, while the conveyor and other structures maybe the same, and the Se in the combustion exhaust gas is made harmless,and modification of the existing combustion exhaust gas treatment systemis easy, and when newly installing this system, the conventional designor equipment may be used as it is.

[0160] The result of combustion exhaust gas treatment experiment by thetest apparatus shown in FIG. 4 is described below. In the experiment,coal containing 3 mg/kg of Se was supplied into a combustion furnace ata rate of 25 kg/h, and the combustion exhaust gas exhausted from thecombustion furnace at a rate of 200 m³ N/h was cooled to 150° C. and fedinto the electrostatic precipitator. In this case, more than 99% of thedust was captured by the electrostatic precipitator, and the amount ofdust collected by the conveyor (the total collected from the hoppers)was 3.4 kg/h. The capturing amount of coarse ash B5 and fine ash B6,mean particle size, and eluting Se concentration (Se componentscontained in captured ash and eluting by the elution test) are shown inTable 5. TABLE 5 Collected ash Item Coarse ash B5 Fine ash B6 Capturingamount kg/h 2.05 1.30 Mean particle size of captured ash 13 5.4 μmEluting selenium concentration in 0.26 0.36 captured ash mg/liter

[0161] That is, in the fine ash B6, the mean particle size was 5.4 μm,and the capturing amount was small, 1.30 kg/h, but the eluting Seconcentration exceeded the standard, 0.36 mg/liter. In coarse ash B5,the mean particle size was 13 μm, the capturing amount was large, 2.05kg/h, but the eluting Se concentration was below the standard, 0.26mg/liter. Accordingly, it is known that the coarse ash B5 can bedirectly discarded. Hence, Se insoluble treatment is not needed in thecoarse ash B5 which is very large in output, and it is evident that therequired amount of treating agent C and required capacity of Se treatingapparatus 125 can be saved substantially.

[0162] The fine ash B6 was fed into the Se treating apparatus 125, andblended with the treating agent C and mixed for 30 minutes, and it wasseparated into solid and liquid by the solid-liquid separator 126, andthe Se concentration was measured in the solid-phase side (dust cake D)and liquid-phase side (filtrate E), and the same results as inembodiment 2 shown in Table 3 were obtained, and the elution standard(0.3 mg/liter) was sufficiently satisfied.

[0163] (Embodiment 4)

[0164]FIG. 5 is a schematic explanatory diagram showing a constitutionof a fourth example of a combustion exhaust gas treatment system of thefirst invention. Same constituent elements as in embodiment 2 in FIG. 3are identified with same reference numerals and their explanations areomitted. This combustion exhaust gas treatment system is, as shown inFIG. 5, characterized by comprising a mixer 161 (mixing means) formixing the dust G undergoing Se insoluble treatment as being fed intothe Se treating apparatus 125 from the conveyor 141 after beingseparated and collected by the hoppers 133, 134 at the outlet side ofthe combustion exhaust gas, and the other dust B1, B2 not undergoing Seinsoluble treating after being separated and collected by the hoppers131, 132 at the inlet side of the combustion exhaust gas, so that thewater content may be 20% or less (preferably 10% or less).

[0165] The Se treating means is an Se treating apparatus 125 a capableof inducing Se insoluble reaction (for example, reaction formulas 1, 2,or 3, 4), by spraying a liquid containing a treating agent such as FeCl₃or Fe₂(SO₄)₃ (concentration about 0.5 to 5.0 wt. %) uniformly into thedust.

[0166] In this example, the dust G undergoing Se insoluble treatment,and the other dust B1, B2 not undergoing Se insoluble treatment aremixed by the mixer 161, and a harmless dust cake H (mixed ash) of lowwater content is formed. Accordingly, without using large-scaleequipment such as solid-liquid separator that requires wastewater(filtrate) treatment, the water content in the dust can be easilylowered, and the harmless dust can be handled easily in waste disposal.In the constitution of the embodiment, the water content of the mixedash H and eluting Se concentration (Se components contained in mixed ashand eluting by the elution test) were measured in the same conditions asin embodiment 2, of which results are shown in Table 6. TABLE 6 FeCl₃Spray Water concentration flow rate of content in Eluting selenium inspray liquid FeCl₃ solution mixed ash concentration in mixed (wt. %)(g/h) (wt. %) ash (mg/liter) 0.5 400 9.7 0.18 1.0 400 10.5 0.15 2.5 40010.6 0.14 5.0 400 9.8 0.16 5.0 600 15.4 0.13 5.0 800 19.9 0.11

[0167] (Embodiment 5)

[0168]FIG. 6 is a schematic explanatory diagram showing a constitutionof a fifth example of a combustion exhaust gas treatment system of thefirst invention. Same constituent elements as in embodiment 4 in FIG. 5are identified with same reference numerals and their explanations areomitted. This combustion exhaust gas treatment system is, as shown inFIG. 6, characterized by comprising a scale forming apparatus 171 (scaleforming means) for compacting the dust H mixed by the mixer 161 to forminto scale.

[0169] In this case, the scale forming apparatus 171 forms the dust Hinto scale dust (scale ash) I, and the volume of the dust H is furtherreduced, and handling in waste disposal is much easier. Besides, thewater content of the dust H is kept under 20% (preferably under 10%) bythe mixer 161, which is effective for making it easier to operate tocompact by the scale forming apparatus 171. As a result of experiment inthe same running conditions as in embodiment 4, the bulk density of thedust cake H (mixed ash) mixed by the mixer 161 was 0.8 g/cc, whereas thebulk density of the dust (scale ash) I formed into scale by the scaleforming apparatus 171 was 1.5 g/cc.

[0170] The first invention may be realized also in other various formsthan the illustrated embodiments. For example, as the treating agent formaking Se insoluble, aside from FeCl₃ or Fe₂(SO₄)₃, chelating agents(e.g. Miyoshi Resin Epolus MX-7), and high polymer heavy metal capturingagents (e.g. Miyoshi Resin Epofloc L-1) can be used. The dust collectingmeans and sorting means of the invention are not limited to theelectrostatic precipitator and single cyclone connected thereto, but thedust collecting means and sorting means of the invention may berealized, for example, by multiple cyclones.

[0171] The combustion exhaust gas treatment system of the firstinvention may be incorporated in part of the combustion exhaust gastreatment system having a conventional desulfurization apparatus. Forexample, in embodiment 1, the desulfurization apparatus for removingsulfurous acid from the combustion exhaust gas sent by the fan 123 maybe provided, for example, as in the prior art shown in FIGS. 24 to 26.Besides, the conventional air heater or gas-gas heater may be used asthe cooling means of the first invention.

[0172] (Effects of the First Invention)

[0173] According to the combustion exhaust gas treatment system of thefirst invention, most of Se in combustion exhaust gas is cooled by thecooling means and condensed, and is removed by the dust collecting meansin a state being contained in the dust. To the dust separated by thedust collecting means, a treating agent is added by the Se treatingmeans, and the existent form of Se in the dust is transformed into aninsoluble compound. Accordingly, if the dust is discarded same as in theprior art, the Se elution standard is satisfied, and Se is made harmlesseasily without requiring complicated aftertreatment.

[0174] By applying Se insoluble treatment only on the dust separated andcollected from the specific recovery unit at the outlet side ofcombustion exhaust gas in the dust collecting means, the required amountof treating agent and required capacity of the Se treating means can bereduced, and the Se is made harmless more easily and economically.

[0175] Also by applying Se insoluble treatment only on the dust of smallparticle size sorted by the sorting means, the required amount oftreating agent and required capacity of the Se treating means can bereduced, and the Se is made harmless more easily and economically.

[0176] By installing mixing means, when the dust undergoing Se insolubletreatment and remaining dust not undergoing Se insoluble treatment aremixed so that the water content may be 20% or less, the water content ofthe dusts may be easily lowered without using large-scale equipment suchas solid-liquid separator that requires treatment of wastewater(filtrate), and therefore handling in disposal of dust may be easier.

[0177] By installing scale forming means, when the dust mixed by themixing means is formed into scale, handling in disposal of dusts may bemuch easier.

[0178] B: Second Invention

[0179] Referring now to drawings, embodiments of the second inventionare described below.

[0180] (Embodiment 1)

[0181]FIG. 7 is a diagram showing an essential structure of a combustionexhaust gas treatment system in embodiment 1 of the second invention.This combustion exhaust gas treatment system comprises, as shown in FIG.7, a cooler 221 (cooling means) for cooling combustion exhaust gas Aexhausted from a coal fired boiler 220, an electrostatic precipitator222 (dust collecting means, sorting means) disposed downstream of thecooler 221, for collecting dust such as fly ash in the combustionexhaust gas A, and separating from the combustion exhaust gas, and a fan223 for supplying the combustion exhaust gas (being rid of dust)discharged from the electrostatic precipitator 222 to a later process.

[0182] At the dust discharge side of the electrostatic precipitator 222,there are, as sequentially arranged, a mixer 225 (mixing means) foradding and mixing Se elution preventive agent C and humidifying liquid Dto the dust, a briquetting machine 226 (scale forming means) for formingthe dust discharged from the mixer 225 into scale, a granulator 227(granulating means) for granulating the scale formed dust (scale dust)formed by the briquetting machine 226 into a size suited to handling,and a screen 228 (sorting means) for screening a proper size of thescale dust granulated by the granulator 227.

[0183] Moreover, a desulfurization apparatus for removing sulfurous acidfrom the combustion exhaust gas sent by the fan 223 may be provided sameas in the prior art shown in FIGS. 24 to 26. Besides, as a matter ofcourse, the conventional air heater or gas-gas heater may be function asthe cooler 221.

[0184] Of the scale dust sorted by the screen 228, scale dust E ofproper size may be directly discarded or recycled, and smaller scaledust F is put back into the mixer 225, and larger scale dust G is sentagain into the granulator 227.

[0185] The cooler 221 is to set the outlet gas temperature, for example,in a range of 150 to 400° C., and the temperature may be set so that thecombustion exhaust gas may be cooled to the temperature for sufficientlycondensing the Se in the combustion exhaust gas. Specifically, thetemperature is set so as to cool the combustion exhaust gas to 350° C.or less, or preferably 310° C. or less.

[0186] The electrostatic precipitator 222 has plural hoppers 231 to 234(recovery units) for separating and collecting dust, and these hoppers231 to 234 are formed sequentially from the inlet side (upstream side)to the outlet side (downstream side) of the combustion exhaust gas, andin such constitution, dust of larger particle size is collected from theinlet side hopper, and dust of smaller particle size is collected fromthe outlet side hopper.

[0187] In this case, only the dust B3, B4 separated and collected fromthe specific hoppers 233, 234 at the outlet side of the combustionexhaust gas, out of the plural hoppers 231 to 234 in the electrostaticprecipitator 222, are led into the mixer 225 to make Se insoluble, whilethe remaining dust B1, B2 are discarded directly.

[0188] The mixer 225 has the function of, in this case, mixing anddischarging the charged dust, Se elution preventive agent C, andhumidifying liquid D, and is provided with, for example, agitationblades inside for sending out the charged matter to the discharge sidewhile agitating it.

[0189] Herein, as the Se elution preventive agent, a chemical reactingwith Se to make it insoluble is necessary, and for example, FeCl₃ orFe₂(SO₄)₃ may be used if at least tetravalent Se (main form: seleniousacid SeO₃ ²⁻) is contained among Se components to be removed.

[0190] If there is hexavalent Se (main form: selenic acid SeO₄ ²⁻) inthe combustion exhaust gas and it must be made insoluble in order toconform to the elution standard, a reducing agent for transforminghexavalent Se into tetravalent Se may be added as treating agent,together with the above chemical. As the reducing agent, for example,sulfurous acid water obtained by blowing SO₂ into water may bepreferably used. When the wet desulfurization apparatus is installed atthe same time, it is preferred to absorb SO₂ by the desulfurizationapparatus, and extract and use the slurry or circulation watercontaining unreacted sulfurous acid.

[0191] The charging amount of the Se elution preventive agent C may beset slightly more than the stoichiometric equivalent for making Se inthe dust completely insoluble determined from the reaction formulas 1,2, or 3, 4.

[0192] As the humidifying liquid D, aside from ordinary industrialwater, the slurry or circulation water of the desulfurization apparatusmay be used, and its charging amount may be a minimum required level forthe ease of handling of dust or for compacting and forming into scale.For example, when the dust amount is 15 t/h, FeCl₃ may be charged byabout 150 kg/h, and humidifying water about 0.79 t/h. Herein, as thehumidifying liquid, by absorbing SO₂ in the desulfurization apparatus,the slurry or circulation water containing unreacted sulfurous acid maybe used, and the dissolved sulfurous acid may act as reducing agent forreducing the hexavalent Se into tetravalent Se.

[0193] Instead of adding the Se elution preventive agent C by chargingtogether with the humidifying liquid D, it may be preliminarily mixed ina solution and added. In such a case, the solution concentration ispreferred to be about 0.05 to 5 wt. %, and the loading may be about 0.5to 10 wt. % to the dust. By so setting, most of Se is made insoluble,and the dust may be handled easily without discharge of filtrate, and itmay be smoothly compacted and formed into scale.

[0194] In thus constituted combustion exhaust gas treatment system, dustremoval treatment and harmless treatment of Se much contained in dustare executed in the following procedure.

[0195] The combustion exhaust gas A released from the boiler 220 isfirst cooled to 350° C. or less by the cooler 221, and therefore atleast at the downstream side of the cooler 221, the Se in combustionexhaust gas is condensed, and mostly deposits on the ash which forms thedust in the combustion exhaust gas. This dust is separated and collectedby the electrostatic precipitator 222, and the dust B1, B2 separated andcollected from the specific hoppers 231, 232 at the inlet side of thecombustion exhaust gas are directly discarded, while only the dust B3,B4 separated and collected from the specific hoppers 233, 234 at theoutlet side of the combustion exhaust gas are fed into the mixer 225.

[0196] In the mixer 225 or subsequent briquetting machine 226, thetetravalent Se (main form: selenious acid SeO₃ ²⁻) contained in thecharged dust reacts with the Se elution preventive agent (FeCl₃ orFe₂(SO₄)₃) as shown in the following reaction formulas 1, 2, or 3, 4,and is made insoluble in a form of iron selenite (Fe₂(SeO₃)₃).

[0197] When hexavalent Se (main form: selenic acid SeO₄ ²⁻) is present,the reducing agent as mentioned above is charged, and this hexavalent Sereacts with the reducing agent to be tetravalent Se, which similarlyreacts as shown in the following reaction formulas 1, 2, or 3, 4 to beinsoluble.

FeCl₃→Fe³⁺+3Cl⁻  (1)

2Fe³⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (2)

or

Fe₂(SO₄)₃→2Fe³⁺+3SO₄ ²⁻  (3)

2Fe³⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (4)

[0198] Accordingly, most of Se is made insoluble and mixed in the scaledust E as iron selenite. Therefore, if the scale dust E may be directlydiscarded in the ash disposal yard, the elution standard can besatisfied. Besides, the dusts B1, B2 separated and collected from thespecific hoppers 231, 232 of the electrostatic precipitator 222 are lowin Se concentration, and if directly discharged in the ash disposalyard, the elution standard is satisfied.

[0199] As described herein, according to the combustion exhaust gastreatment system of embodiment 1, without requiring complicatedaftertreatment of the prior art, the Se in combustion exhaust gas can beremoved together with dust, and finally most of it is made insoluble andpresent in the scale dust E so as to be discarded directly. Still more,because of the constitution for adding a humidifying liquid to mix thedust and Se elution preventive agent, and forming into scale, ascompared with the constitution for forming the dust into slurry, mixinginsoluble treating agent, and separating into solid and liquid todiscard, it does not require large-scale equipment or facility such aswastewater (filtrate) treatment apparatus and solid-liquid separator,and handling of dust in waste disposal is much easier. If notparticularly needed in waste disposal, the scale forming process may beomitted, so that the system may be further simplified.

[0200] In this case, moreover, only the dust B3, B4 separated andcollected from the specific hoppers 233, 234 at the outlet side of thecombustion exhaust gas are presented to Se insoluble treatment, andtherefore the required amount of Se elution preventive agent C, andrequired capacity of the mixer 225 (mixing means) and briquettingmachine 226 (scale forming means) can be reduced, and the Se is madeharmless more easily and inexpensively. That is, as mentioned above,since more Se is contained (deposited) on the dust (ash) of smallerparticle size separated and collected from the specific recovery unit atthe outlet side, only by applying the insoluble treatment on the dust ofsmaller particle size, the Se is made harmless on the whole, therebycontributing to reduction of facility cost and running cost.

[0201] The result of dust collecting experiment by using theelectrostatic precipitator in the system of FIG. 7 is described below.In the experiment, coal containing 3 mg/kg of Se was supplied in acombustion furnace at a rate of 25 kg/h, and combustion exhaust gasexhausted from the combustion furnace at a rate of 200 m³ N/h was cooledto 150° C., and fed into the electrostatic precipitator. In this case,more than 99% of the dust was captured by the electrostatic precipitator(Se capturing rate about 99.4%), and the total amount of dust collectedfrom the hoppers was 3.4 kg/h. The discharge amount of dust (collectedash) collected from the hoppers 231, 232, or 233, 234, the mean particlesize, and eluting Se concentration are shown in Table 7. Herein, theeluting Se concentration refers to the concentration of Se in the dustanalyzed according to the elution test and atomic absorption methodconforming to the ordinance No. 13 of Environmental Agency of Japan, andit shows the amount of Se contained in the dust and eluting in theelution test. TABLE 7 Collected ash Collected ash Collected ashdischarged from discharged from hoppers 231, 232 hoppers 233, 234(combustion (combustion exhaust gas exhaust gas Item inlet side) outletside) Discharge amount (kg/h) 2.27 1.14 Mean particle size of collected12 5 ash (μm) Selenium eluting concentration 0.20 0.49 in collected ash(mg/liter)

[0202] More specifically, the discharge amount of the dust B3, B4separated and collected from the hoppers 233, 234 at the outlet side ofthe combustion exhaust gas was slight, 1.14 kg/h, but the eluting Seconcentration was 0.49 mg/liter, high above the landfill standard (0.3mg/liter). On the other hand, the discharge amount of the dust B1, B2separated and collected from the hoppers 231, 232 at the inlet side ofcombustion exhaust gas was large, 2.27 kg/h, but the eluting Seconcentration was 0.20 mg/liter, far below the standard. Accordingly, itis known that the dust B1, B2 separated and collected from the hoppers231, 232 at the inlet side of the combustion exhaust gas can be directlydiscarded. That is, Se insoluble treatment is not needed in the dust atthe inlet side of the combustion exhaust gas which is about twice largerin the discharge amount, and hence it is evident that the requiredamount of the Se elution preventive agent C and the required capacity ofthe mixer 225 can be substantially saved.

[0203] Incidentally, such difference in eluting Se concentration isregarded to be due to the particle size of dust (ash). That is, whengaseous Se (SeO₂) is condensed and deposits on the surface of the ashforming the dust, ash of smaller particle size is greater in thespecific surface area per unit weight, and hence more Se deposits.

[0204] On the other hand, in the dust collector such as theelectrostatic precipitator mentioned above, coarse ash particles arelikely to be captured at the inlet side of the combustion exhaust gas,and fine ash particles are likely to be captured at the outlet side ofthe combustion exhaust gas, and in other words there is a sortingfunction, and it is hence considered that the eluting Se concentrationis high in the dust captured at the outlet side of the combustionexhaust gas.

[0205] (Embodiment 2)

[0206] As other embodiment of the second invention, embodiment 2 isdescribed below. Same constituent elements as in embodiment 1 areidentified with same reference numerals and their explanations areomitted.

[0207]FIG. 8 shows an essential structure of a combustion exhaust gastreatment system of embodiment 2. The combustion exhaust gas treatmentsystem of this embodiment comprises a sorter 251 (sorting means) forclassifying the dust B1 to B4 captured by the electrostatic precipitator222 and conveyed in batch into large particle size (coarse ash) B5 andsmall particle size (fine ash) B6, and only fine ash B6 sorted by thesorter 251 is captured by a fine particle capturing apparatus 252, andfed into a mixer 225 to make Se insoluble. In this case, the dust B1 toB4 conveyed in batch are conveyed by air H and led into the sorter 251.The sorter 251 may be constituted by, for example, a cyclone, and it isconvenient when it is designed to adjust the degree of sorting. As thefine particle capturing apparatus 252, in this case, a bag filter isused.

[0208] In this case, only the fine ash B6 is subjected to Se insolubletreatment, and same as in embodiment 1, therefore, the required amountof Se elution preventive agent C and required capacity of mixer 225 canbe reduced, so that the Se may be made harmless more easily andinexpensively.

[0209] In this combustion exhaust gas treatment system, only by addingthe mixer 225 and sorter 251 to the conventional combustion exhaust gastreatment system shown in FIGS. 24 to 26, the system can be realized,while the conveyor and other structures for conveying the dust capturedby the electrostatic capacitor 222 may be the same, and the Se in thecombustion exhaust gas is made harmless, and modification of theexisting combustion exhaust gas treatment system is easy, and when newlyinstalling this system, the conventional design or equipment may be usedas it is.

[0210] The result of dust collecting experiment by the electrostaticprecipitator in the structure of the system in FIG. 8 is describedbelow. In the experiment, coal containing 3 mg/kg of Se was suppliedinto a combustion furnace at a rate of 25 kg/h, and the combustionexhaust gas exhausted from the combustion furnace at a rate of 200 m³N/h was cooled to 150° C. and fed into the electrostatic precipitator.In this case, more than 99% of the dust was captured by theelectrostatic precipitator, and the total amount of dust collected fromthe hoppers was 3.4 kg/h. The capturing amount of coarse ash B5 and fineash B6, mean particle size, and eluting Se concentration are shown inTable 8. TABLE 8 Collected ash Item Coarse ash B5 Fine ash B6 Capturingamount kg/h 2.05 1.30 Mean particle size of captured ash 13 5.4 μmEluting selenium concentration in 0.26 0.36 captured ash mg/liter

[0211] That is, in the fine ash B6, the mean particle size was 5.4 μm,and the capturing amount was small, 1.30 kg/h, but the eluting Seconcentration exceeded the standard, 0.36 mg/liter. In coarse ash B5,the mean particle size was 13 μm, the capturing amount was large, 2.05kg/h, but the eluting Se concentration was below the standard, 0.26mg/liter. Accordingly, it is known that the coarse ash B5 can bedirectly discarded. Hence, Se insoluble treatment is not needed in thecoarse ash B5 which is very large in output, and it is evident that therequired amount of Se elution preventive agent C and required capacityof the mixer 225 can be saved substantially.

[0212] The second invention may be realized also in other various formsthan the illustrated embodiments. For example, as the Se elutionpreventive agent, aside from FeCl₃ or Fe₂(SO₄)₃, chelating agents (e.g.Miyoshi Resin Epolus MX-7), and high polymer heavy metal capturingagents (e.g. Miyoshi Resin Epofloc L-1) can be used. The dust collectingmeans and sorting means of the invention are not limited to theelectrostatic precipitator and single cyclone connected thereto, but thedust collecting means and sorting means of the invention may berealized, for example, by multiple cyclones.

[0213] The combustion exhaust gas treatment system of the secondinvention may be incorporated in part of the combustion exhaust gastreatment system having a conventional desulfurization apparatus. Forexample, in embodiment 1, the desulfurization apparatus for removingsulfurous acid from the combustion exhaust gas sent by the fan 223 maybe provided, for example, as in the prior art shown in FIGS. 24 to 26.Besides, the conventional air heater or gas-gas heater may be used asthe cooling means of the second invention.

[0214] Moreover, all of the dust separated and collected by theelectrostatic precipitator 222 may be treated by introducing into themixer 225 (mixing means).

[0215] (Effects of the Second Invention)

[0216] According to the combustion exhaust gas treatment system of thesecond invention, most of Se in combustion exhaust gas is cooled by thecooling means and condensed, and is removed by the dust collecting meansin a state being contained in the dust. To the dust separated by thedust collecting means, Se elution preventive agent and humidifyingliquid or a solution of Se elution preventive agent is added by themixing means, and the existent form of Se in the dust is transformedinto an insoluble compound. Accordingly, if the dust is discarded sameas in the prior art, the Se elution standard is satisfied, and Se ismade harmless easily without requiring complicated aftertreatment.Moreover, by adding the humidifying liquid and Se elution preventiveagent, or spraying a solution of Se elution preventive agent to mix withthe dust, and forming the dust into scale, as compared with the case offorming the dust into slurry, mixing Se insoluble treating agent, andseparating into solid and liquid to discard, it does not require largescale equipment or facility such as wastewater (filtrate) treatingfacility or solid-liquid separator, and handling is much easier in wastedisposal of dust.

[0217] Also by feeding only the dust separated and collected from thespecific recovery unit at the outlet side of the combustion exhaust gasin the dust collecting means or the dust of small particle size sortedby the sorting means into the mixing means to make insoluble, therequired amount of Se elution preventive agent and required capacity ofthe mixing means and scale forming means can be reduced, and the Se ismade harmless more easily and economically.

[0218] C: Third Invention:

[0219] Embodiments of the third invention is described below whilereferring to the accompanying drawings.

[0220] (Embodiment 1)

[0221]FIG. 9 is a schematic structural (principle) diagram showing anexample of a combustion exhaust gas treatment system of the thirdinvention (1) to (3), (6) and (9), and FIG. 10 is a structural diagramspecifically showing the constitution of the combustion exhaust gastreatment system. In the following explanation, the combustion exhaustgas to be treated is supposed to contain both hexavalent Se andtetravalent Se. In the combustion exhaust gas treatment system in theembodiment, as shown in FIG. 9, dust containing Se is removed from thecombustion exhaust gas 310 by an electrostatic precipitator 305, andpart of the dust (ash) removed by the electrostatic precipitator 305 isheated by heating means 311 making use of upstream gas of air heater orthe like to sublimate Se in the dust (hexavalent Se and tetravalent Se),thereby adsorbing in an adsorption column 312.

[0222] On the other hand, the remaining dust is repulped (dissolved inwater) by the liquid from a wet desulfurization apparatus 320 (ORPcontrol: hereinafter desulfurization apparatus 320) by repulping means313, and formed into slurry, and a treating agent A (such as FeCl₃) fromtreating agent feeding means 316 is charged by mixing means 314, and byseparating into solid and liquid in separating means 315, tetravalent Seis solidified and separated into the solid-phase side. On the otherhand, hexavalent Se dissolved in the liquid-phase side in the separatingmeans 315 is led into the desulfurization apparatus 320, and nearly thewhole volume is reduced by the so-called ORP control(oxidation-reduction potential control) to be transformed intotetravalent Se, thereby facilitating the treatment by makingwastewater-free in the wastewater treating apparatus 350.

[0223] The heating means 311 is designed to heat the ash introduced fromthe electrostatic precipitator 305 up to a temperature for sublimatingand gasifying hexavalent Se and tetravalent Se (100 to 1200° C.,preferably 320 to 1000° C.), and part of produced gas is led into theabsorption column 312, and the rest is sent into the combustion exhaustgas feed-in side of the desulfurization apparatus 320. The ash left overin the heating means 311 is deprived of Se components, and is recycledas cement material. The adsorption column 312 is designed to absorb andcapture part of Se in the produced gas from the heating means 311 so asto be solidified, and herein the gas containing Se which is not adsorbedis sent into the combustion exhaust gas feed-in side of thedesulfurization apparatus 320.

[0224] To the repulping means 313, as shown in FIG. 10, the liquid in afiltrate tank 333 is supplied by means of a pump 334 of thedesulfurization apparatus 320, and the dust introduced from theelectrostatic precipitator 305 is made into slurry by this liquid. Themixing means 314 is, for example, composed of a mixing tank and anagitating mechanism for agitating the liquid in the mixing tank, and thedust slurry formed by the repulping means 313, the treating agent A fromthe treating agent feeding means 316, and, if necessary, a filteradditive B are charged, and they are mixed and sent into the separatingmeans 315 at the downstream side. As the treating agent A, a chemicalreacting at least with tetravalent Se (main form: selenious acid SeO₃²⁻) to make insoluble is needed, and, for example, FeCl₃ or Fe₂(SO₄)₃may be used. As the filter additive B, a chemical large in particle sizeand having a function for supporting solid-liquid separation is desired,and, for example, gypsum (gypsum C produced in the desulfurizationapparatus 320) may be used. The separating means 315 is, for example, acentrifugal setting machine, and, in this case, only the filtrate isreturned to the absorbent slurry column 335 of the desulfurizationapparatus 320.

[0225] The desulfurization apparatus 320 is of tank oxidation type, andcomprises an absorption column 321 for feeding an absorbent slurry(composed of limestone in this example) into a bottom tank 322, acirculation pump 323 for sending the absorbent slurry in the tank 322into an upper part 321 a (combustion exhaust gas feed-in unit) of theabsorption tank 321 to contact with the combustion exhaust gas, a rotaryarm type air sparger 324 supported in the tank 322 for rotationhorizontally by means of a motor not shown, and agitating the slurry inthe tank 322 and blowing in the supplied aid efficiently into the tank322 as fine bubbles, and an air feed tube 325 for feeding air into thisair sparger 324, and it is designed to obtain gypsum by totallyoxidizing by efficient contact between air and the absorbent slurryabsorbing sulfurous acid in the tank 322.

[0226] A slurry pump 331 for sucking out the slurry in the tank 322 isconnected to the tank 322, and the slurry sucked out by this slurry pump331 is concentrated through a thickener not shown, and is supplied intothe solid-liquid separator 332 to be filtered, and the gypsum C in theslurry is taken out as solid cake (usually water content of about 10%).On the other hand, the separated water by the thickener and the filtrate(mainly water) from the solid-liquid separator 332 are once sent intothe filtrate tank 333, and, as required, makeup water D or return liquidE from the wastewater treating apparatus 350 is added, and part of suchliquid is sent into the absorbent slurry tank 335 by a pump 334, andmixed with limestone F (CaCO₃) supplied from a limestone silo not shownto be formed into an absorbent slurry, which is supplied again into thetank 322 by a slurry pump 336.

[0227] The desulfurization apparatus 320 is further provided with, as apreferred embodiment of the third invention, oxidation-reductionreaction control means 340 for controlling the oxidation-reductionreaction in the absorption column 321. This oxidation-reduction reactioncontrol means 340 is composed of a sensor 341 disposed in the dischargeside piping of the circulation pump 323 for detecting theoxidation-reduction potential of the slurry in the tank 322, a flow ratecontrol valve 342 disposed in the midst of the air feed tube 325 foradjusting the air feed rate into the air sparger 324, and a controller343 for controlling the action of the flow rate control valve 342 on thebasis of the detection output of the sensor 341. Herein, the sensor 341is composed by immersing an electrode, for example, made of platinuminto slurry. The controller 343 is designed to control the openingdegree of the flow rate control valve 342 continuously, so that the airfeed rate into the air sparger 324 may be a minimum required limit foroxidizing and digesting the sulfurous acid dissolved in the slurry fromthe combustion exhaust gas. For example, more specifically, on the basisof the correlation of the concentration of sulfurous acid andoxidation-reduction potential, the oxidation-reduction potential whenthe concentration of sulfurous acid is nearly zero is predetermined asthe reference potential, and, by proportional control, when theoxidation-reduction potential detected by the sensor 341 becomes lowerthan this reference potential, the air feed rate is increased accordingto the deviation, and when the oxidation-reduction potential detected bythe sensor 341 becomes higher than this reference potential, the airfeed rate is decreased according to the deviation.

[0228] Incidentally, since the oxidation-reduction reaction controlmeans 340 is designed to feed a minimum required limit for oxidizing thetotal volume of sulfurous acid, it eventually has a function of inducinga newly total reduction reaction of the other acids contained in theslurry by the sulfurous acid.

[0229] That is, in this case, as mentioned later, the slurry suppliedinto the tank 322 from the absorbent slurry tank 335 contains hexavalentSe (main form: selenic acid SeO₄ ²⁻), but by the control of thecontroller 343, it reacts with the sulfurous acid absorbed from thecombustion exhaust gas to undergo reduction reaction to be transformedinto tetravalent Se (main form: selenious acid SeO₃ ²⁻), which takesplace in the absorption column 321. This reaction is expressed in thefollowing reaction formula (5).

SeO₄ ²⁻+SO₃ ²⁻→SeO₃ ²⁻+SO₄ ²⁻  (5)

[0230] In this embodiment, the wastewater treating apparatus 350 is aso-called wastewater-free treating apparatus in a known constitutioncomprising a pretreatment facility 351, an electric dialysis facility352, a secondary concentrating facility 353, and a solidifying facility354. In this wastewater treating apparatus 350, part of the liquid inthe filtrate tank 333 is supplied by the pump 334 of the desulfurizationapparatus 320, and impurities in this liquid (for example, Se and Cl)are removed mainly by the function of the electric dialysis facility352, and the residue after removal is returned to the filtrate tank 333or absorbent slurry tank 335 of the desulfurization apparatus 320. Theremoved impurities are finally solidified in the solidifying facility354, but at least prior to the solidifying process (for example, at aprior stage of the secondary concentrating facility 353), the treatingagent A for reacting with tetravalent Se (main form: selenious acid SeO₃²⁻) to make it insoluble is mixed into the removed impurities from thetreating agent feeding means 316.

[0231] In thus constituted combustion exhaust gas treatment system,first, the sulfurous acid in the combustion exhaust gas after removingthe dust, and gasified Se are removed, and gypsum is produced andcollected in the following procedure.

[0232] That is, the combustion exhaust gas introduced into theabsorption column 321 (including the gas sent from the heating means311) contacts with the absorbent slurry sprayed form a header pipe 326by the circulation pump 323, and the sulfurous acid and gasified Se areabsorbed and removed, and discharged as treated combustion exhaust gasfrom a combustion exhaust gas lead-out unit 321 b.

[0233] The sulfurous acid absorbed in the absorbent slurry sprayed fromthe header pipe 326 and flowing down through a filler 327 is agitated bythe air sparger 324 in the tank 322, and contacts with multiple bubblesto be oxidized, and further undergoes neutralization reaction to becomegypsum. In the absorption column 321, by the reaction in the reactionformula (5), nearly whole volume of hexavalent Se (main form: selenicacid SeO₄ ²⁻) is transformed into tetravalent Se (main form: seleniousacid SeO₃ ²⁻). Principal reactions taking place in this process (otherthan reaction formula (5) are expressed in reaction formulas (6) to (8).

[0234] (Absorption Column Combustion Exhaust Gas Lead-in Part)

SO₂+H₂O→H⁺+HSO₃ ⁻  (6)

[0235] (Tank)

H⁺+HSO₃ ⁻+1/2O₂→2H⁺+SO₄ ²⁻  (7)

2H⁺+SO₄ ²⁻+CaCO₃+H₂O→CaSO₄.2H₂O+CO₂  (8)

[0236] Thus, in the tank 322, gypsum (CaSO₄.2H₂O), a slight amount oflimestone (CaCO₃), and tetravalent Se (main form: selenious acid SeO₃²⁻) are suspended or dissolved, and they are sucked out by the slurrypump 331, and concentrated through a thickener not shown, and suppliedinto the solid-liquid separator 332 to be filtered, and gypsum C isobtained in a cake form of low water content (usually water contentabout 10%). At this time, meanwhile, tetravalent Se (main form:selenious acid SeO₃ ²⁻), if not dissolved partly, may be slightlyseparated and mixed into the gypsum C, but is mostly sent into thefiltrate tank 333 together with the separated water or filtrate.

[0237] Consequently, the dust removal treatment in the combustionexhaust gas and the treating action of Se contained much in the dust inthis combustion exhaust gas treatment system are explained below.

[0238] Part of dust (ash) removed by the electrostatic precipitator 305is heated by the heating means 311 to be gasified, and is partlyadsorbed and solidified as mentioned above, while the rest is sent andtreated in the absorption column 321 of the desulfurization apparatus320. On the other hand, the remainder of the dust removed by theelectrostatic precipitator 305 is repulped (dissolved in water) in theliquid sent from the filtrate tank 333 of the desulfurization apparatus320 by the repulsing means 313 to be formed into slurry, and treatingagent A and, if necessary, filter additive B are added and mixed by themixing means 314.

[0239] At this time, in the mixing means 314, the tetravalent Se (mainform: selenious acid SeO₃ ²⁻) contained in the slurry formed by therepulping means 313 (dust slurry) reacts with the treating agent A (e.g.FeCl₃ or Fe₂(SO₄)₃) in the following reaction formulas (1), (2), or (3),(4), and becomes insoluble in a form of iron selenite (Fe₂(SeO₃)₃).

FeCl₃→Fe₃++3Cl⁻  (1)

2Fe³⁻+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (2)

[0240] or

Fe₂(SO₄)₃→2Fe³⁺+3SO₄ ²⁻  (3)

2Fe³⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (4)

[0241] Accordingly, when the dust slurry is separated into solid andliquid by the separating means 315, the tetravalent Se is separated inthe solid-phase side as iron selenite, and mixed in insoluble form intothe dust cake G discharged from the separating means 315. On the otherhand, the hexavalent Se in the dust slurry is dissolved in theliquid-phase side and is contained in the filtrate, and is mixed, inthis case, into the absorbent slurry tank 335 of the desulfurizationapparatus 320. In the separating means 315, by the function of thefilter additive B (gypsum, etc.) charged in the mixing means 314,effective dehydration is realized, and a low water content in the dustcake G is achieved. At the same time, the hexavalent Se contained in thefiltrate in the separating means 315 and mixed in the adsorbent slurrytank 335 of the desulfurization apparatus 320 is also contained in theabsorbent slurry and sent into the absorption column 321 of thedesulfurization apparatus 320 with the pump 336, and therefore almostall volume thereof is transformed into tetravalent Se in the reaction(reaction formula (5).

[0242] The function of the wastewater treating apparatus 350 in thiscombustion exhaust gas treatment system is described below. As mentionedabove, mainly hexavalent Se is sequentially mixed into the slurrysolution circulating in the desulfurization apparatus 320 as beingcontained in the separated water of the dust slurry, and when thishexavalent Se enters the absorption column 321, it is almost completelytransformed into tetravalent Se by the above reaction (reaction formula5), and in stationary state, therefore, much tetravalent Se mainlyformed by reduction of hexavalent Se is present in the slurry solutioncirculating in the desulfurization apparatus 320. In this case, thewastewater treating apparatus 350 functions to remove this tetravalentSe, like other impurities (e.g. Cl), so as not to be accumulatedexcessively in the slurry solution circulating in the desulfurizationapparatus 320.

[0243] That is, in the wastewater treating apparatus 350, part of theslurry solution circulating in the desulfurization apparatus 320 isextracted from the discharge side of the pump 334, and the impurities inthis solution (Se, Cl, etc.) are removed mainly by the function of theelectric dialysis facility 352, and returned to the filtrate tank 333 ofthe desulfurization apparatus 320. The removed impurities are mixed withthe treating agent A charged from the treating agent feeding means 316,and concentrated in the secondary concentration apparatus 353,solidified by the solidifying facility 354, and discarded in the ashdisposal yard or the like as impurity chip H. At this time, thetetravalent Se in the impurities reacts with the treating agent A in theformulas (1), (2), or (3), (4), and is transformed into iron selenite(Fe₂(SeO₃)₃), and is present in an insoluble form in the impurity chipH.

[0244] As described herein, according to the combustion exhaust gastreatment system of the embodiment, together with the conventionalpurification of combustion exhaust gas (removal of dust, removal ofsulfurous acid), Se in the combustion exhaust gas is removed along withdust, and finally it is contained, in an insoluble form, in the dustcake G or impurity chip H, so as to be discarded. Moreover, hexavalentSe which is hard to be treated (made insoluble) is transformed intotetravalent Se which is easy to discard, by the treating agent by theoxidation-reduction reaction control means 340 in the absorption column321 of the desulfurization apparatus 320, and therefore, as comparedwith the system comprising an independent reaction column fortransforming hexavalent Se into tetravalent Se, for example, Se incombustion exhaust gas may be removed and made harmless easily andinexpensively.

[0245] Still more, as the solvent for repulsing the dust captured by theelectrostatic precipitator 305 by the repulsing means 313, by using partof the circulation liquid of the desulfurization apparatus 320, thewater flow (circulation volume) and consumption are saved as comparedwith the system of feeding water separately into the repulping means313, and the running cost of the system is curtailed, and at the sametime the required capacity of the wastewater treating apparatus 350 isdecreased, so that the facility cost may be further reduced.

[0246] When the mixing means 314 is designed to charge filter additiveB, the dehydration rate is heightened in the separating means 315, anddust cake G of lower water content is formed, so that carrying orhandling of dust cake G may be easier.

[0247] Moreover, according to this combustion exhaust gas treatmentsystem, by the function of the oxidation-reduction reaction controlmeans 340, nearly all of hexavalent Se is eventually transformed intotetravalent Se, and is made insoluble and discarded, and hence theconcentration of hexavalent Se remaining in the dust cake G or impuritychip H (not made insoluble) is very slight, and the elution standard issatisfied with a sufficient margin.

[0248] (Embodiment 2)

[0249]FIG. 11 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the third invention (4) and(9). Same constituent elements as in embodiment 1 are identified withsame reference numerals, and their explanations are omitted. In thecombustion exhaust gas treatment system of the embodiment, as shown inFIG. 11, the dust captured by the electrostatic precipitator 305 isdirectly introduced into the absorption column 321 of thedesulfurization apparatus 320, and the treating agent A is charged intothe slurry extracted from the absorption column 321 of thedesulfurization apparatus 320 by the treating agent feeding means 316 inthe mixing means 314.

[0250] In this case, all Se in the dust once enters the absorptioncolumn 321 except for the portion extracted by the heating means 311,and in the absorption column 321, too, the hexavalent Se is transformedinto tetravalent Se, and this tetravalent Se is made insoluble by thetreating agent A, and is mixed into the gypsum C or the impurity chip Hin the wastewater treating apparatus 350. Herein, the absorption column321 also functions as the repulping means 313 in embodiment 1 and thesolid-liquid separator 332 also functions as the separating means 315 inembodiment 1, and as compared with the system in embodiment 1,therefore, the repulping means 313 and separating means 315 are notnecessary, thereby contributing further to reduction of the facilitycost.

[0251] In this embodiment, meanwhile, although the cost may be furthersaved as compared with embodiment 1, but due to the effect of massivedust (impurities) mixing into the absorption column, it may be difficultto keep a high desulfurization rate or high quality of gypsum C, andwhere this problem is feared, embodiment 1 or embodiment 4 or 5mentioned later may be preferable, and in this respect the constitutionof embodiment 1 is superior. Incidentally, the treating agent A may bemixed into other position than the position shown in FIG. 11, as far aswithin the slurry system of the desulfurization apparatus 320, or may bedirectly mixed into the absorption column 321.

[0252] (Embodiment 3)

[0253]FIG. 12 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the third invention (5) and(9). Same constituent elements as in embodiment 1 are identified withsame reference numerals, and their explanations are omitted. In thecombustion exhaust gas treatment system of the embodiment, as shown inFIG. 12, without installing electrostatic precipitator 305, combustionexhaust gas is directly fed into the absorption column 321 of thedesulfurization apparatus 320, together with fly ash and dust, and thetreating agent A is charged into the slurry extracted from theabsorption column 321 of the desulfurization apparatus 320 from thetreating agent feeding means 316 in the mixing means 314.

[0254] In this case, all Se in the dust once enters the absorptioncolumn 321, and in the absorption column 321, the hexavalent Se istransformed into tetravalent Se, and this tetravalent Se is madeinsoluble by the treating agent A, and is mixed into the gypsum C or theimpurity chip H in the wastewater treating apparatus 350. Herein, theabsorption column 321 also functions as the electrostatic precipitator305 and repulsing means 313 in embodiment 1, and the solid-liquidseparator 332 also functions as the separating means 315 in embodiment1, and as compared with the system in embodiment 1, therefore, theelectrostatic precipitator 305, repulping means 313 and separating means315 are not necessary, thereby contributing further to reduction of thefacility cost.

[0255] In this embodiment, meanwhile, although the cost may be furthersaved as compared with embodiment 1, but due to the effect of massivedust (impurities) mixing into the absorption column, it may be difficultto keep a high desulfurization rate or high quality of gypsum C, andwhere this problem is feared, embodiment 1 or embodiment 4 or 5mentioned later may be preferable, and in this respect the constitutionof embodiment 1 is superior.

[0256] Yet, since the electrostatic precipitator 305 is not provided,the byproduct obtained in the heating means 311 is slight, and wherethis problem is feared, the constitution of embodiment 1, 2 or 4 may bepreferred, and in this respect the constitution of embodiment 1 or othermay be superior. In this constitution, too, the mixing position of thetreating agent A is not limited to the position shown in FIG. 12, but itmay be mixed in any arbitrary position in the slurry system of thedesulfurization apparatus 320, or may be directly charged into theabsorption column 321.

[0257] (Embodiment 4)

[0258]FIG. 13 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the third invention (7) and(9). Same constituent elements as in embodiment 1 are identified withsame reference numerals, and their explanations are omitted. In thecombustion exhaust gas treatment system of the embodiment, as shown inFIG. 13, a desulfurization apparatus 360 having a cooling and dustcollecting column 361 for cooling the dust and removing dust is disposedat the upstream side of the absorption column 321, and the dust capturedby the electrostatic precipitator 305 is directly fed into the coolingand dust collecting column 361 of the desulfurization apparatus 360,while the treating agent A is charged into the slurry extracted from thecooling and dust removing column 361 from the treating agent feedingmeans 316 in the mixing means 314. Herein, in the cooling and dustcollecting column 361, the liquid from the filtrate tank 333 is suppliedfrom the pump 334, and this liquid is sprayed from an upper header pipe363 by a circulation pump 362. Between the cooling and dust collectingcolumn 361 and absorption column 321, a mist eliminator, not show, isprovided.

[0259] In this case, all Se in dust once gets into the cooling and dustremoving column 361, and the reaction in (reaction formula (5)) occursin the cooling and dust collecting column 361, and hexavalent Se isalmost totally transformed into tetravalent Se, and this tetravalent Seis made insoluble by the treating agent A, and is mixed into the dustcake G or impurity chip H. Herein, the cooling and dust collectingcolumn 361 also functions as the repulsing means 313 in embodiment 1,and therefore the repulsing means 313 is not needed as compared withembodiment 1, and the facility cost may be reduced.

[0260] In this embodiment, moreover, different from embodiments 1 to 3,massive dust does not mix into the absorption column 321, and thereforethe facility cost may be further reduced, and the high desulfurizationrate may be kept and gypsum C of high quality may be realized.

[0261] In FIG. 13, meanwhile, the separated water of the separatingmeans 315 is directly fed into the wastewater treating apparatus 350,but for further perfect transformation reaction from hexavalent Se intotetravalent Se, the separated water may be fed, for example, into theabsorbent slurry tank 335 and then guided into the absorption column321.

[0262] The treating agent A may be also charged directly into thecooling and dust collecting column 361.

[0263] (Embodiment 5)

[0264]FIG. 14 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the third invention (8) and(9). Same constituent elements as in embodiment 1 are identified withsame reference numerals, and their explanations are omitted. In thecombustion exhaust gas treatment system of the embodiment, as shown inFIG. 14, a desulfurization apparatus 360 having a cooling and dustcollecting column 361 for cooling the dust and removing dust is disposedat the upstream side of the absorption column 321, and the combustionexhaust gas not being rid of dust is directly fed into the cooling anddust collecting column 361 of the desulfurization apparatus 360, whilethe treating agent A is charged into the slurry extracted from thecooling and dust removing column 361 from the treating agent feedingmeans 316 in the mixing means 314.

[0265] In this case, all Se in dust once gets into the cooling and dustremoving column 361, and the reaction in (reaction formula (5)) occursin the cooling and dust collecting column 361, and hexavalent Se isalmost totally transformed into tetravalent Se, and this tetravalent Seis made insoluble by the treating agent A, and is mixed into the dustcake G or impurity chip H. Herein, the cooling and dust collectingcolumn 361 also functions as the electrostatic precipitator 305 andrepulping means 313 in embodiment 1, and therefore the electrostaticprecipitator 305 and repulsing means 313 are not needed as compared withembodiment 1, and the facility cost may be reduced.

[0266] In this embodiment, moreover, different from embodiments 2 and 3,massive dust does not mix into the absorption column 321, and thereforethe facility cost may be further reduced, and the high desulfurizationrate may be kept and gypsum C of high quality may be realized.

[0267] In FIG. 14, meanwhile, the separated water of the separatingmeans 315 is directly fed into the wastewater treating apparatus 350,but for further perfect transformation reaction from hexavalent Se intotetravalent Se, the separated water may be fed, for example, into theabsorbent slurry tank 335 and then guided into the absorption column321.

[0268] Yet, since the electrostatic precipitator 305 is not provided,the byproduct obtained in the heating means 311 is slight, and wherethis problem is feared, the constitution of embodiment 1, 2 or 4 may bepreferred, and in this respect the constitution of embodiment 1 or othermay be superior.

[0269] The third invention may be also realized in many other formsaside from the foregoing embodiments. For example, if hexavalent Se doesnot exist and only tetravalent Se is present in the combustion exhaustgas, the process or apparatus for reducing hexavalent Se intotetravalent Se is not needed. The process and apparatus for heating andrecycling part of the dust removed by the dust collector by heatingmeans may be provided only where necessary.

[0270] The constitution of the desulfurization apparatus is not limitedto the tank oxidation type shown in the embodiments, and, for example,an oxidation column in which the slurry extracted from the absorptioncolumn is fed may be separately installed, and by blowing air into thisoxidation column, final oxidation-reduction reaction may be performedherein. In this case, too, hexavalent Se is transformed into tetravalentSe in the absorption column or oxidation column.

[0271] The repulping means and mixing means in embodiment 1 are notalways required to be composed of independent tank, but, for example, itmay be also designed to repulp the dust and mix the treating agent inone tank (that is, the repulping means and mixing means of the inventionmay be composed of a single tank).

[0272] In embodiments 2 to 5, the charging position of the treatingagent A may be a position in the wastewater treating apparatus 350. Thatis, since the slurry liquid in the absorption column or cooling and dustcollecting column of the desulfurization apparatus is circulating in thewastewater treating apparatus 350, the entire Se can be made insolubleby charging the treating agent only in the wastewater treating apparatus350.

[0273] (Effects of the Third Invention)

[0274] According to the combustion exhaust gas treatment system of thethird invention (1), if hexavalent Se is contained in the combustionexhaust gas, all Se (both hexavalent and tetravalent) removed from thecombustion exhaust gas as dust can be finally discharged as tetravalentSe, and the Se elution standard may be satisfied easily only by treatingwith the treating agent to be insoluble, and the absorption column ofthe desulfurization apparatus also functions as the reduction reactionfacility of hexavalent Se, and the facility constitution of the entiresystem is facilitated as compared with the constitution of installing anindependent reaction column for reducing Se. According to the combustionexhaust gas treatment system of the third invention (2), since part ofthe circulation liquid of the desulfurization apparatus can be used assolvent in the repulping means, the water flow (circulation) andconsumption can be saved as compared with the constitution for feedingwater separately.

[0275] According to the combustion exhaust gas treatment system of thethird invention (3), since filter additive is charged into the mixingmeans or separating means, the dehydration performance in the separatingmeans is enhanced, the solid matter (dust cake) of low water content andeasy to handle is obtained.

[0276] According to the combustion exhaust gas treatment system of thethird invention (4), the Se elution standard may be easily satisfied,and the desulfurization apparatus functions also as hexavalent Sereduction reaction facility or as dust repulping means, so that theconstitution of the entire system is simplified as compared with thesystem comprising Se reduction reaction column or repulping meansseparately.

[0277] According to the combustion exhaust gas treatment system of thethird invention (5), the Se elution standard may be easily satisfied,and the desulfurization apparatus functions also as dust collector, orhexavalent Se reduction reaction facility, or dust repulsing means, sothat the constitution of the entire system is simplified as comparedwith the system comprising dust collector, Se reduction reaction columnor repulping means separately.

[0278] According to the combustion exhaust gas treatment system of thethird invention (6), the oxidation-reduction reaction control meanscontrols the oxidation-reduction reaction of the slurry in thedesulfurization apparatus, so that the hexavalent Se mixed in the slurryin the desulfurization apparatus may be almost completely reduced intotetravalent form by the sulfurous acid in the slurry. Accordingly, ifhexavalent Se is contained in the combustion exhaust gas, thishexavalent Se can be almost completely transformed into tetravalent formin the desulfurization apparatus, so that the Se in the combustionexhaust gas may be made insoluble more easily and perfectly.

[0279] According to the combustion exhaust gas treatment system of thethird invention (7), the Se elution standard may be easily satisfied,and moreover since the desulfurization apparatus functions also ashexavalent Se reducing reaction facility, the constitution of the entiresystem is simplified as compared with the system comprising reactioncolumn for reducing Se separately. Also in this case, since the coolingand dust removing column also function as dust repulping means, theconstitution of the entire system is more simplified as compared withthe system comprising repulping means separately. More preferably, sincemuch dust (Se and other impurities) does not mix into the slurry in theabsorption column of the desulfurization apparatus, the performance suchas desulfurization rate in the desulfurization apparatus can bemaintained high.

[0280] According to the combustion exhaust gas treatment system of thethird invention (8), the Se elution standard may be easily satisfied,and the desulfurization apparatus functions also as dust collector, orhexavalent Se reduction reaction facility, or dust repulping means, sothat the constitution of the entire system is simplified as comparedwith the system comprising dust collector, Se reduction reaction columnor repulping means separately. More preferably, since much dust (Se andother impurities) does not mix into the slurry in the absorption columnof the desulfurization apparatus, the performance such asdesulfurization rate in the desulfurization apparatus can be maintainedhigh.

[0281] According to the combustion exhaust gas treatment system of thethird invention (9), the Se eluting into wastewater in thedesulfurization apparatus can be also made insoluble, and more perfectSe insoluble treatment is realized, and Se-free gypsum can be collected.Depending on the treating conditions, moreover, the insoluble treatingagent may be charged only into the wastewater treating apparatus, sothat the entire system can be simplified.

[0282] D: Fourth Invention:

[0283] Referring now to the drawings, embodiments of the fourthinvention are described below.

[0284] (Embodiment 1)

[0285]FIG. 15 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system according to the fourthinvention (1) to (3),and (5) to (7).

[0286] In the combustion exhaust gas treatment system of thisembodiment, as shown in FIG. 15, the dust containing Se is removed by anelectrostatic precipitator 470, the dust (ash) removed by theelectrostatic precipitator 470 is heated by heating means 411 tosublimate and gasify the Se in the dust, and this gas is fed into a wetdesulfurization apparatus 420 (hereinafter, desulfurization apparatus420), together with the combustion exhaust gas being rid of dust, to betreated.

[0287] In the desulfurization apparatus, as shown in reaction formulas(9) and (10), tetravalent Se (main form: selenious acid SeO₃ ²⁻) andhexavalent Se (main form: selenic acid SeO₄ ²⁻) are present. Thishexavalent Se is almost completely reduced in the desulfurizationapparatus 420 by so-called ORP control (oxidation-reduction potentialcontrol) to be transformed into tetravalent Se.

SeO_(2(g))+H₂O→2H⁺+SeO₃ ²⁻  (9)

SeO₃ ²⁻+1/2O₂→SeO₄ ²⁻  (10)

[0288] Herein, the heating means 411 is, for example, a kiln, which isdesigned to heat the ash B3, B4 introduced from the electrostaticprecipitator 470 until Se is sublimated and gasified by the supplied hotair I, and produced gas M is fed into the absorption column 421 of thedesulfurization apparatus together with the combustion exhaust gas,while the remaining ash J is taken out to be recycled as cement materialor the like. As means for feeding hot air I into the heating means 411,in this case, a heavy oil fired boiler 412 is provided, and by burningheavy oil K and heating the air L, hot air I at about, for example,1000° C. is supplied.

[0289] The treating temperature in the heating means 411 may be 100 to1200° C., but in order to gasify the Se in the dust more perfectly andefficiently, it is preferred to set in a range of 320 to 1000° C. Forinstance, when the flow and temperature of the dust introduced into theheating means 411 from the electrostatic precipitator 470 are 15 t/h and90° C., to heat to about 320° C., the flow and temperature of hot air I,and the kiln specification for composing the heating means 411 may beset, for example, as follows. That is, the flow rate of hot air I is3100 m³ N/h, temperature of hot air I is 1000° C., the filling rate inthe kiln is 8%, the ash density in kiln is 0.5, and the residence timein the kiln is 0.5 hr.

[0290] The electrostatic precipitator 470 comprises plural hoppers 471to 474 for separating and collecting the dust, and these hoppers 471 to474 are formed sequentially from the inlet side (upstream side) to theoutlet side (downstream side) of the combustion exhaust gas, and in thisconstitution, dust of larger particle size is collected from the inletside hopper, and dust of smaller particle size is collected from theoutlet side hopper. In this case, only the dust B3, B4 separated andcollected from the specific hoppers 473, 474 at the outlet side are fedinto the heating means 411, and the remaining dust B1, B2 are recycledas cement material or discarded directly. In this embodiment, meanwhile,the electrostatic precipitator 470 functions as the dust collectingmeans and sorting means of the fourth invention.

[0291] The desulfurization apparatus 420 is of tank oxidation type inthis embodiment, and comprises an absorption tank 421 in which anabsorbent slurry (composed of limestone in this case) is supplied into abottom tank 422, a circulation pump 423 for sending the absorbent slurryin the tank 422 into an upper part 421 a (combustion exhaust gas lead-inpart) of the absorption tank 421 to contact with combustion exhaust gas,an arm-rotary type air sparger 424 supported in the tank 422 forrotating horizontally by a motor not shown, agitating the slurry in thetank 422, and blowing the supplied air efficiently into the tank 422 asfine bubbles, and an air feed pipe 425 for feeding air into the airsparger 424, in which the absorbent slurry absorbing sulfurous acid andthe air are brought into contact efficiently in the tank 422, and thewhole volume is oxidized to obtain gypsum.

[0292] In this tank 422, a slurry pump 431 for sucking out the slurry inthe tank 422 is connected, and the treating agent A is charged by themixing means 414 into the slurry sucked out by this slurry pump 431, andthe mixture is supplied into a solid-liquid separator 432 to befiltered, and the gypsum C in the slurry is taken out as solid cake(usually water content about 10%). On the other hand, the filtrate(mainly water) from the solid-liquid separator 432 is once sent into thefiltrate tank 433, and, if necessary, makeup water D or return liquid Efrom the wastewater treating apparatus 450 is added, and part of theliquid is sent into the adsorbent slurry tank 435 by the pump 434, andmixed with limestone F (CaCO₃) supplied from limestone silo not shown,and is supplied as absorbent slurry again into the tank 422 by theslurry pump 436. The mixing means 414 is composed of, for example, amixing tank and an agitating mechanism for agitating the liquid in themixing tank. As the treating agent A, a chemical at least reacting withtetravalent Se (main form: selenious acid SeO₃ ²⁻) to make it insolubleis needed, and for example, FeCl₃ or Fe₂(SO₄)₃ may be used.

[0293] The desulfurization apparatus 420 is further equipped withoxidation-reduction reaction control means 440 for controlling theoxidation-reduction reaction in the absorption column 421. Theoxidation-reduction reaction control means 440 consists, in this case,of a sensor 441 provided in the discharge side piping of the circulationpump 423 for detecting the oxidation-reduction potential of the slurryin the tank 422, a flow control valve 442 provided on the way of airfeed pipe 425 for adjusting the air supply flow into the air sparger424, and a controller 443 for controlling the action of the flow controlvalve 442 on the basis of the detection output of the sensor 441.Herein, the sensor 441 is realized by immersing an electrode made of,for example, platinum in the slurry. The controller 443 is designed tocontrol continuously the opening degree of the flow control valve 442,so that the air feed rate into the air sparger 424 may be a minimumrequired limit for oxidizing and digesting the sulfurous acid dissolvedin the slurry from the combustion exhaust gas. More specifically, on thebasis of the correlation of the sulfurous acid concentration andoxidation-reduction potential, the oxidation-reduction potential whenthe sulfurous acid concentration is nearly zero is predetermined as thereference potential, and the proportional control is effected toincrease the air feed rate depending on the deviation when theoxidation-reduction potential detected by the sensor 441 is lower thanthis reference potential, and to decrease the air feed rate depending onthe deviation when the oxidation-reduction potential detected by thesensor 441 is higher than the reference potential.

[0294] Incidentally, since the oxidation-reduction reaction controlmeans 440 is designed to supply air of the minimum required limit foroxidizing the total volume of sulfurous acid, it eventually has afunction of inducing the reaction for reducing almost whole volume ofother acids contained in the slurry by the sulfurous acid.

[0295] That is, the gas M containing vaporized Se released from theheating means 411 is fed into the absorption column 421 together withcombustion exhaust gas, and becomes tetravalent Se (main form: seleniousacid SeO₃ ²⁻) and hexavalent Se (main form: selenic acid SeO₄ ²⁻), butby the control of the controller 443, the hexavalent Se reacts with thesulfurous acid absorbed from the combustion exhaust gas to becometetravalent Se (main form: selenious acid SeO₃ ²⁻) in the reductionreaction, which takes place in the absorption column 421. This reactionis expressed in the following reaction formula (11).

SeO₄ ²⁻+SO₃ ²⁻→SeO₃ ²⁻+SO₄ ²⁻  (11)

[0296] The wastewater treating apparatus 450 is a so-calledwastewater-free treating apparatus, comprising a pretreatment facility451, an electric dialysis facility 452, a secondary concentratingfacility 453, and a solidifying facility 454. In this wastewatertreating apparatus 450, part of the liquid in the filtrate tank 433 issupplied by the pump 434 of the desulfurization apparatus 420, andimpurities in this liquid (for example, Cl) are removed mainly by thefunction of the electric dialysis facility 452, and the residue afterremoval is returned to the filtrate tank 433 or absorbent slurry tank435 of the desulfurization apparatus 420. The removed impurities arefinally solidified in the solidifying facility 454, but at least priorto the solidifying process (for example, at a prior stage of thesecondary concentrating facility 453), the treating agent A for reactingwith tetravalent Se (main form: selenious acid SeO₃ ²⁻) to make itinsoluble is mixed in.

[0297] In thus constituted combustion exhaust gas treatment system, thecombustion exhaust gas is sufficiently cooled upstream of theelectrostatic precipitator 470, and Se in the combustion exhaust gas ismostly condensed and deposits on the fly ash or other dust (particularlyon small particles), and therefore most Se in combustion exhaust gas iscaptured once by the electrostatic precipitator 470 together with thedust. In this case, of the captured dust B1 to B4, dust B1 and B2 oflarge particle size are small in Se content, and may be hence directlyused as cement material or discarded, while only dust B3 and B4 of smallparticle size are heated in the heating means 411, and the Se in thedust B3, B4 is gasified, and fed into the absorption column 421 of thedesulfurization apparatus 420 together with the combustion exhaust gasreleased from the electrostatic precipitator 470.

[0298] The combustion exhaust gas introduced into the absorption column421 (including the gas sent from the heating means 411 and others)contacts with the absorbent slurry sprayed from a spray valve 426 by thecirculation pump 423, and the sulfurous acid and gasified Se areabsorbed and removed, and is discharged from the combustion exhaust gaslead-out part 421 b as treated combustion exhaust gas.

[0299] The sulfurous acid sprayed from the spray valve 426 and absorbedin the absorbent slurry flowing down through a filler 427 is agitated bythe air sparger 424 in the tank 422 and contacts with multiple bubblesblown in to be oxidized, and further undergoes neutralization reactionto become gypsum. In the absorption column 421, by the reaction of thereaction formula (11), nearly whole volume of hexavalent Se (main form:selenic acid SeO₄ ²⁻) is transformed into tetravalent Se (main form:selenious acid SeO₃ ²⁻). Principal reactions (except for reactionformula 11) taking place in this process are expressed in the followingreaction formulas (12) to (14).

[0300] (Absorption Column Combustion Exhaust Gas Lead-in Part)

SO₂+H₂O→H⁺+HSO₃ ⁻  (12)

[0301] (Tank)

H⁺+HSO₃ ⁻+½O₂→2H⁺+SO₄ ²⁻  (13)

2H⁺+SO₄ ²⁻+CaCO₃+H₂O→CaSO₄.2H₂O+CO₂  (14)

[0302] Thus, in the tank 422, gypsum (CaSO₄.2H₂O), a small amount oflimestone (CaCO₃) as absorbent, and mainly tetravalent Se (main form:selenious acid SeO₃ ²⁻) are suspended, and they are sucked out by theslurry pump 431, and the treating agent A is mixed by the mixing means414, and the mixture is supplied into the solid-liquid separator 432 tobe filtered, and gypsum C is obtained as cake form of low water content(usually water content about 10%).

[0303] Most of tetravalent Se (main form: selenious acid SeO₃ ²⁻)undergoes the reaction expressed in reaction formulas 1, 2, or 3, 4, andis made insoluble in a form of iron selenite (Fe₂(SeO₃)₃), and is mixedin the separated gypsum c.

FeCl₃→Fe³⁺+3Cl⁻  (1)

2Fe³⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (2)

[0304] or

Fe₂(SO₄)₃→2Fe³⁺+3SO₄ ²⁻  (3)

2Fe³⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (4)

[0305] If, however, it is not desired that iron selenite (Fe₂(SeO₃)₃) ismixed in the separated and collected gypsum C, the slurry from theslurry pump 431 is supplied directly into the solid-liquid separator 432through a line 437 (shown in FIG. 15), and gypsum C of high purity isrecovered. In this case, Se is treated to be insoluble in the wastewatertreating apparatus 450 shown below.

[0306] The function of the wastewater treating apparatus 450 in thecombustion exhaust gas treatment system is described below.

[0307] As mentioned above, the vaporized Se is absorbed in the slurry inthe desulfurization apparatus 420 together with the sulfurous acid inthe combustion exhaust gas, and the hexavalent Se thereof reacts(reaction formula (11) in the absorption column 421 and is almostcompletely transformed into tetravalent Se. This tetravalent Se istreated same as other impurities (e.g. Cl) in the wastewater treatingapparatus 450, and this Se and other impurities are removed so as not tobe accumulated excessively in the slurry solution circulating in thedesulfurization apparatus 420.

[0308] That is, in the wastewater treating apparatus 450, part of thefiltrate of the filtrate tank 433 in the desulfurization apparatus 420is extracted from the discharge side of the pump 434, and the impuritiesin this solution (Cl, etc.) are removed mainly by the function of theelectric dialysis facility 452, and returned to the filtrate tank 433 ofthe desulfurization apparatus 420. The liquid leaving the electricdialysis facility 452 is mixed with the treating agent, and the mixtureis concentrated in the secondary concentration apparatus 453, solidifiedby the solidifying facility 454, and discarded in the ash disposal yardor the like as impurity chip H. At this time, the tetravalent Se in theimpurities reacts with the treating agent A in the formulas (1), (2), or(3), (4), and is transformed into iron selenite (Fe₂(SeO₃)₃), and ispresent in an insoluble form in the impurity chip H.

[0309] As described herein, according to the combustion exhaust gastreatment system of the embodiment, in addition to the conventionalpurification of combustion exhaust gas (removal of dust, removal ofsulfurous acid), Se in the combustion exhaust gas is removed along withdust, and finally it is contained, in an insoluble form, in the dustcake G or impurity chip H, so as not to be eluted when recycled ordiscarded. Moreover, hexavalent Se which is hard to be treated (madeinsoluble) is transformed into tetravalent Se which is easy to discard,by the treating agent by the oxidation-reduction reaction control means440 in the absorption column 421 of the desulfurization apparatus 420,and therefore, as compared with the system comprising an independentreaction column for transforming hexavalent Se into tetravalent Se, forexample, Se in combustion exhaust gas may be removed and made harmlesseasily and inexpensively.

[0310] What is more, according to the combustion exhaust gas treatmentsystem, by the function of the oxidation-reduction reaction controlmeans 440, nearly whole volume of hexavalent Se is eventuallytransformed into tetravalent Se in the absorption column 421, and isfinally made insoluble and discarded, and therefore the concentration ofSe (not made insoluble) remaining in the gypsum cake C or impurity chipH is trifling, and the elution standard may be satisfied with an amplemargin.

[0311] In this case, moreover, by applying Se insoluble treatment onlyon the dust B3, B4 separated and collected from the specific hoppers473, 474 at the outlet side of the combustion exhaust gas in theelectrostatic precipitator 470, the required amount of the treatingagent A and the required capacity of the heating means 411 may bereduced, so that the Se may be made harmless more easily andinexpensively.

[0312] That is, according to the study by the present inventors, it isknown that more Se is contained (deposited) in the smaller particle sizedust (ash) separated and collected from the specific recovery unit atthe outlet side, and the Se is made harmless on the whole only byapplying insoluble treatment on the dust of smaller particle size,thereby contributing to reduction of facility cost and running cost.

[0313] Below are explained the results of dusts heating experiment, dustcapturing test, and elution experiment, in the same apparatus as in theabove embodiment.

[0314] The heating experiment was conducted by heating the dustcontaining 84 mg/kg of Se at various temperatures (200 to 1200° C.) forvarious durations (5 to 30 minutes). Before and after heatingexperiment, the Se elution test of dust was conducted in a methodconforming to ordinance No. 13 of the Environmental Agency of Japan, andthe Se concentration in the elution solution was analyzed by the atomicabsorption photometry by the hydrogen compound generating method.

[0315] The results by heating temperature are shown in FIG. 18, andeffects of h eating time are given in FIG. 19. When heated for 10 to 30minutes at temperature of 320° C. or more, the Se elution in the dustwas less than the elution standard of 0.3 mg/liter concerning thelandfill regulation, and it is known that Se is mostly gasified. Attemperature of 200° C., by extending the heating time to 30 minutes, theSe elution from the dust was below the elution standard concerninglandfill.

[0316] Therefore, when gasifying the Se in dust by heating, as thetemperature not allowing the gasified Se to condense again, by heatingthe dust temperature to 100 to 1200° C., preferably 320 to 1000° C., theSe in the dust can be removed. That is, in this embodiment, the dust Jafter being heated by the heating means 411 is proved to be recycled ordiscarded directly.

[0317] In the dust capturing and elution test, coal containing 3 mg/kgof Se was supplied into a combustion furnace at a rate of 25 kg/h, andthe combustion exhaust gas exhausted at 200 m³ N/h from the combustionfurnace was cooled to 150° C., and fed into the electrostaticprecipitator. In this case, more than 99% of the dust was captured bythe electrostatic precipitator, and the total volume collected from allhoppers was 3.4 kg/h. Discharge, mean particle size, and eluting Seconcentration in the dust B1, B2 (collected ash) collected from theupstream hoppers 471, 472, and dust B3, B4 collected from downstreamhoppers 473, 474 were measured, of which results are shown in Table 9.TABLE 9 Collected ash Collected ash discharged Collected ash dischargedfrom hoppers 471, 472 from hoppers 473, 474 (combustion exhaust(combustion exhaust gas Item gas inlet side) outlet side) Dischargeamount kg/h 2.27 1.14 Mean particle size of 12 5 collected ash μmEluting selenium 0.20 0.49 concentration in collected ash mg/liter

[0318] More specifically, the discharge amount of the dust B3, B4separated and collected from the hoppers 473, 474 at the outlet side ofthe combustion exhaust gas was slight, 1.14 kg/h, but the eluting Seconcentration was 0.49 mg/liter, high above the standard. On the otherhand, the discharge amount of the dust B1, B2 separated and collectedfrom the hoppers 471, 472 at the inlet side of combustion exhaust gaswas large, 2.27 kg/h, but the eluting Se concentration was 0.20mg/liter, far below the standard. Accordingly, it is known that the dustB1, B2 separated and collected from the hoppers 471, 472 at the inletside of the combustion exhaust gas can be directly discarded. That is,Se insoluble treatment is not needed in the dust at the inlet side ofthe combustion exhaust gas which is about twice larger in the dischargeamount, and hence it is evident that the required amount of the treatingagent A and the required capacity of the heating means 411 can besubstantially saved.

[0319] Incidentally, such difference in eluting Se concentration isregarded to be due to the particle size of dust (ash). That is, whengaseous Se (SeO₂) is condensed and deposits on the surface of the ashforming the dust, ash of smaller particle size is greater in thespecific surface area per unit weight, and hence more Se deposits. Onthe other hand, in the dust collector such as the electrostaticprecipitator mentioned above, coarse ash particles are likely to becaptured at the inlet side of the combustion exhaust gas, and fine ashparticles are likely to be captured at the outlet side of the combustionexhaust gas, and in other words there is a sorting function, and it ishence considered that the eluting Se concentration is high in the dustcaptured at the outlet side of the combustion exhaust gas.

[0320] In this embodiment, due to the effect of Se mixing into theabsorption column, it may be difficult to realize a high quality(purity) of gypsum C, and if this is a problem, as mentioned above, Seinsoluble treatment may be done only in the wastewater treatingapparatus 450.

[0321] Incidentally, the treating agent A may be mixed in other positionthan the position shown in FIG. 15 as far as within the slurry system ofthe desulfurization apparatus 420, or may be directly charged into theabsorption column 421. Or the treating agent A may be mixed only in thewastewater treating apparatus 450 of the desulfurization apparatus 420,and hence the mixing means 414 in FIG. 15 may be omitted. In this case,in the desulfurization apparatus 420, all Se (especially tetravalent Se)circulates as being dissolved in the slurry solution, and part of the Seis sequentially led into the wastewater treating apparatus 450 to bemade insoluble, and hence does not mix into the gypsum C, so that it isbeneficial when desired to keep a high purity of gypsum.

[0322] Moreover, if it is not necessary to reduce the charging amount ofthe treating agent A or required capacity of the heating means 411, allof the dust B1 to B4 collected in the electrostatic precipitator 470 maybe fed into the heating means 411 and treated.

[0323] (Embodiment 2)

[0324]FIG. 16 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the fourth invention (4).Same constituent elements as in embodiment 1 are identified with samereference numerals, and their explanations are omitted.

[0325] In the combustion exhaust gas treatment system of the embodiment,as shown in FIG. 16, a desulfurization apparatus 460 having a coolingand dust collecting column 461 for cooling the dust and removing dust isdisposed at the upstream side of the absorption column 421, and the gasincluding Se generated in heating means 411, together with combustionexhaust gas, is fed into the cooling and dust collecting column 461 ofthe desulfurization apparatus 460, while the treating agent A is chargedinto the slurry extracted from the cooling and dust removing column 461by the mixing means 414. and it is separated into solid and liquid bythe separating means 415.

[0326] Herein, in the cooling and dust collecting column 461, the liquidfrom the filtrate tank 433 is supplied from the pump 434, and thisliquid is sprayed from an upper header pipe 463 by a circulation pump462. Between the cooling and dust collecting column 461 and absorptioncolumn 421, a mist eliminator, not show, is provided.

[0327] In this case, the Se separated from the dust by heating once getsinto the cooling and dust removing column 461, and the reaction in(reaction formula (11)) occurs in the cooling and dust collecting column461, and hexavalent Se is almost totally transformed into tetravalentSe, and this tetravalent Se is made insoluble by the treating agent A,and is mixed into the dust cake G or impurity chip H in the wastewatertreating apparatus 450. Herein, different from embodiment 1, fine dustnot captured by the electrostatic precipitator does not mix into theabsorption column 421, and therefore a high desulfurization rate may bekept and gypsum C of high quality may be easily obtained.

[0328] In FIG. 16, meanwhile, the separated water of the separatingmeans 415 is directly fed into the wastewater treating apparatus 450,but for further perfect transformation reaction from hexavalent Se intotetravalent Se, the separated water may be fed, for example, into theabsorbent slurry tank 435 and then guided into the absorption column421.

[0329] The treating agent A may be also charged directly into thecooling and dust collecting column 461, or same as in embodiment 1, thetreating agent A may be charged only into the wastewater treatingapparatus 450, and the mixing means 414 and separating means 415 may beomitted.

[0330] (Embodiment 3)

[0331]FIG. 17 is a schematic structural diagram showing an example of acombustion exhaust gas treatment system of the fourth invention (6).Same constituent elements as in embodiment 1 are identified with samereference numerals and their explanations are omitted.

[0332] This combustion exhaust gas treatment system, as shown in FIG.17, comprises a sorter 481 (sorting means) for classifying the dust B1to B4 captured by the electrostatic precipitator 470 and conveyed inbatch into large particle size (coarse ash) B5 and small particle size(fine ash) B6, and only fine ash B6 sorted by the sorter 481 is capturedby a fine particle capturing apparatus 482, and fed into the heatingmeans 411.

[0333] Meanwhile, in this case, the dust B1 to B4 are conveyed in batchby air N and led into the sorter 481. The sorter 481 may be constitutedby, for example, a cyclone, and it is convenient when it is designed toadjust the degree of sorting. As the fine particle capturing apparatus482, in this case, a bag filter is used.

[0334] In this case, only the fine ash B6 is gasified and fed into thedesulfurization apparatus 420 to be made, and therefore, same as inembodiment 1, the required amount of treating agent A and requiredcapacity of heating means 411 can be reduced, so that the Se may be madeharmless more easily and inexpensively.

[0335] In this combustion exhaust gas treatment system, only by addingthe heating means and sorter to the conventional combustion exhaust gastreatment system (capable of conveying the dust from the electrostaticprecipitator in bath), the combustion exhaust gas treatment system canbe realized without modifying the conveyor for conveying the dust fromthe electrostatic precipitator and other structures, and the Se in thecombustion exhaust gas is made harmless, and modification of theexisting combustion exhaust gas treatment system is easy, and when newlyinstalling this system, the conventional design or equipment may be usedas it is.

[0336] The result of dust heating experiment, dust capturing test, andelution experiment by the same apparatus as in the embodiment isdescribed below.

[0337] In the experiment, coal containing 3 mg/kg of Se was suppliedinto a combustion furnace at a rate of 25 kg/h, and the combustionexhaust gas exhausted from the combustion furnace at a rate of 200 m³N/h was cooled to 150° C. and fed into the electrostatic precipitator.In this case, more than 99% of the dust was captured by theelectrostatic precipitator, and the amount of dust collected by theconveyor (the total collected from the hoppers) was 3.4 kg/h. Thecapturing amount of coarse ash B5 and fine ash B6, mean particle size,and eluting Se concentration are shown in Table 10. TABLE 10 Collectedash Item Coarse ash B5 Fine ash B6 Capturing amount kg/h 2.05 1.30 Meanparticle size of captured ash 13 5.4 μm Eluting selenium concentrationin 0.26 0.36 captured ash mg/liter

[0338] That is, in the fine ash B6, the mean particle size was 5.4 μm,and the capturing amount was small, 1.30 kg/h, but the eluting Seconcentration exceeded the standard, 0.36 mg/liter. In coarse ash B5,the mean particle size was 13 μm, the capturing amount was large, 2.0kg/h, but the eluting Se concentration was below the standard, 0.26mg/liter. Accordingly, it is known that the coarse ash B5 can bedirectly discarded. Hence, Se insoluble treatment is not needed in thecoarse ash B5 which is very large in output, and it is evident that therequired amount of treating agent A and required capacity of heatingmeans 411 can be saved substantially.

[0339] The fourth invention may be also realized in many other formsaside from the foregoing embodiments. For example, if hexavalent Se doesnot exist and only other Se than hexavalent is present in the combustionexhaust gas in the desulfurization apparatus, the process or apparatusfor reducing hexavalent Se into tetravalent Se is not needed.

[0340] The constitution of the desulfurization apparatus is not limitedto the tank oxidation type shown in the embodiments, and, for example,an oxidation column in which the slurry extracted from the absorptioncolumn is fed may be separately installed, and by blowing air into thisoxidation column, final oxidation-reduction reaction may be performedherein. In this case, too, hexavalent Se is transformed into tetravalentSe in the absorption column or oxidation column.

[0341] In the embodiments, as mentioned above, the charging position ofthe treating agent A may be a position in the wastewater treatingapparatus 450. That is, since the slurry liquid in the absorption columnor cooling and dust collecting column of the desulfurization apparatusis circulating in the wastewater treating apparatus 450, the entire Secan be made insoluble by charging the treating agent only in thewastewater treating apparatus 450.

[0342] As the treating agent for making tetravalent Se insoluble, forexample, aside from FeCl₃, Fe₂(SO₄)₃, chelating agent (e.g. Epolus MX-7of Miyoshi Resin), or high molecular heavy metal capturing agent (e.g.Epofloc L-1 of Miyoshi Resin) may be used.

[0343] (Effects of the Fourth Invention)

[0344] According to the combustion exhaust gas treatment system of thefourth invention (1), most of Se in flue is removed by the dustcollector in a state being contained in the dust, and is heated andgasified by the heating means. Accordingly, almost no Se is left over inthe dust after treatment, which can be directly recycled or discarded,and the Se elution standard is easily satisfied.

[0345] According to the combustion exhaust gas treatment system of thefourth invention (2), the Se removed together with the dust from thecombustion exhaust gas is heated and gasified, and introduced into thedesulfurization apparatus, and eventually all of hexavalent Se can betreated as tetravalent Se, and only by insoluble treatment by treatingagent, the Se elution standard is easily satisfied, and moreover theabsorption column of the desulfurization apparatus also functions as theSe reducing reaction facility, and hence the constitution of the entiresystem is simple as compared with the constitution for providing thereaction column for reducing Se separately. Still more, owing to theconstitution designed for separating Se from the dust by heating meansand feeding into the desulfurization apparatus, so that the entire dustmay not fed into the desulfurization apparatus, recycling of dust iseasy, and lowering of desulfurization performance in the desulfurizationapparatus can be avoided.

[0346] According to the combustion exhaust gas treatment system of thefourth invention (3), the oxidation-reduction reaction control meanscontrols the oxidation-reduction reaction of the slurry in thedesulfurization apparatus, so that the hexavalent Se mixed in the slurryin the desulfurization apparatus may be almost entirely reduced by thesulfurous acid in the slurry to be tetravalent. Hence, the hexavalent Secan be almost completely transformed into tetravalent form in thedesulfurization apparatus, and Se insoluble treatment in the combustionexhaust gas is done more easily and perfectly.

[0347] According to the combustion exhaust gas treatment system of thefourth invention (4), the Se elution standard can be satisfied easily,and the desulfurization apparatus functions also as hexavalent Sereducing reaction facility, and therefore the constitution of the entiresystem is simplified as compared with the constitution for comprisingreaction column for reducing Se separately. In this case, the gasifiedSe and combustion exhaust gas are fed and treated in the cooling anddust collecting column in the desulfurization apparatus, and not onlydust but also impurities such as Se may not be mixed passively into theslurry in the absorption column of the desulfurization apparatus, andtherefore the desulfurization rate in the desulfurization apparatus ismaintained high, and gypsum of high quality can be obtained.

[0348] According to the combustion exhaust gas treatment system of thefourth invention (5), only the dust separated and collected from thespecific recovery unit at the outlet side of the combustion exhaust gasin the dust collecting means is heated, and only the gasified Se is fedinto the desulfurization apparatus to be made insoluble, and thereforethe required amount of treating agent and required capacity of heatingmeans may be reduced, so that the Se may be made harmless more easilyand economically.

[0349] According to the combustion exhaust gas treatment system of thefourth invention (6), only the dust of small particle size sorted by thesorting means is heated, and only the gasified Se is fed into thedesulfurization apparatus to be made insoluble, and therefore therequired amount of treating agent and required capacity of heating meansmay be reduced, so that the Se may be made harmless more easily andeconomically.

[0350] According to the combustion exhaust gas treatment system of thefourth invention (6), the heating temperature of the dust by the heatingmeans is 100 to 1200° C., and hence the gasified Se will not becondensed again in the dust, so that the Se can be removed efficientlyfrom the dust, and therefore the Se elution standard of dust can besatisfied easily.

[0351] E. Fifth Invention:

[0352] Embodiments of the fifth invention are described below byreference to the drawings.

[0353] (Embodiment 1)

[0354] Embodiment 1 of the fifth invention (1) and (3) is described inthe first place.

[0355]FIG. 20 is a schematic structural diagram showing a constitutionof a combustion exhaust gas treatment system in embodiment 1 of thefifth invention. Constituent elements same as in the prior art areidentified with same reference numerals, and their explanations areomitted. In the combustion exhaust gas treatment system of thisembodiment, as shown in FIG. 20, circulation liquid B (liquid dischargedfrom a pump 534 described below) extracted from a wet combustion exhaustgas desulfurization apparatus 520 (hereinafter desulfurization apparatus520) is mixed with treating agent A by mixing means 513, and thecirculation liquid B mixed with the treating agent A is sprayed by apump 514 from a spray pipe 516 a or 516 b provided in a combustionexhaust gas lead-in passage 515 upstream of an electrostaticprecipitator 505, and the Se contained in the dust is made insoluble bythe treating agent A, discharged as dust E, while the wastewaterdischarged from the desulfurization apparatus 520 is evaporated toeliminate the wastewater. Herein, the pump 514, spray pipe 516 a or 516b, and the piping system for connecting them compose the spray means ofthe fifth invention.

[0356] In this embodiment, since the electrostatic precipitator 505 isprovided, as described below, most Se in combustion exhaust gas isremoved as being contained in the dust, and very little Se mixes intothe desulfurization apparatus 520, but in the desulfurization apparatus,as shown in reaction formulas 9 and 10, there are tetravalent Se (mainform: selenious acid SeO₃ ²⁻) and hexavalent Se (main form: selenic acidSeO₄ ²⁻). In this constitution, the hexavalent Se is almost completelyreduced by the so-called ORP control (oxidation-reduction potentialcontrol) in the desulfurization apparatus 520, and is transformed intotetravalent Se.

SeO_(2(g))+H₂O→2H⁺+SeO₃ ²⁻  (9)

SeO₃ ²⁻+½O₂→SeO₄ ²⁻  (10)

[0357] The mixing means 513 is composed of, for example, a mixing tank,and an agitating mechanism for agitating the liquid in the mixing tank.The circulation liquid B extracted from the desulfurization apparatus520 and the treating agent A are charged and mixed, and extracted by thepump 514, and sent into the spray pipe 516 a or 516 b. Herein, as thetreating agent, a chemical at least reacting with tetravalent Se (mainform: selenious acid SeO₃ ²⁻) to make it insoluble is needed, and forexample, FeCl₃ or Fe₂(SO₄)₃ may be used.

[0358] The spray pipe 516 a or 516 b is composed of a pipe main bodyinstalled in the combustion exhaust gas lead-in passage (duct) 515 andconnected to the pump 514, and a spray nozzle formed in the pipe mainbody. The location of the spray pipe 516 a or 516 b (the position forspraying the liquid) in FIG. 20 is in the upstream of a heat recoveryunit 504 and in the upstream of the electrostatic precipitator 505, butit may be also installed in the upstream side of an air heater, notshown, installed in the upstream side of the combustion exhaust gaslead-in passage 515, as far as the sprayed liquid may splash in thecombustion exhaust gas and be gasified by the heat of the combustionexhaust gas, and the treating agent A in the liquid may efficientlycontact with the dust in the combustion exhaust gas.

[0359] The desulfurization apparatus 520 is of tank oxidation type, andcomprises an absorption column 521 for feeding an absorbent slurry(composed of limestone in this example) into a bottom tank 522, acirculation pump 523 for sending the absorbent slurry in the tank 522into an upper part 521 a (combustion exhaust gas feed-in unit) of theabsorption tank 521 to contact with the combustion exhaust gas, a rotaryarm type air sparger 524 supported in the tank 522 for rotationhorizontally by means of a motor not shown, and agitating the slurry inthe tank 522 and blowing in the supplied aid efficiently into the tank522 as fine bubbles, and an air feed tube 525 for feeding air into thisair sparger 524, and it is designed to obtain gypsum by totallyoxidizing by efficient contact between air and the absorbent slurryabsorbing sulfurous acid in the tank 522.

[0360] A slurry pump 531 for sucking out the slurry in the tank 522 isconnected to the tank 522, and the slurry sucked out by this slurry pump531 is supplied into a solid-liquid separator 532 to be separated intosolid and liquid, and gypsum C in the slurry is taken out as solid cake(usually water content of about 10%). On the other hand, the separatedliquid (mainly water) from the solid-liquid separator 532 are once sentinto a separated liquid tank 533, and, as required, makeup water D isadded, and part of such liquid is sent into an absorbent slurry tank 535by a pump 534, and mixed with limestone F (CaCO₃) supplied from alimestone silo not shown to be formed into an absorbent slurry, which issupplied again into the tank 522 by a slurry pump 536.

[0361] The desulfurization apparatus 520 is further provided with, as apreferred embodiment of the fifth invention, oxidation-reductionreaction control means 540 for controlling the oxidation-reductionreaction in the absorption column 521. This oxidation-reduction reactioncontrol means 540 is composed of a sensor 541 disposed in the dischargeside piping of the circulation pump 523 for detecting theoxidation-reduction potential of the slurry in the tank 522, a flow ratecontrol valve 542 disposed in the midst of the air feed tube 525 foradjusting the air feed rate into the air sparger 524, and a controller543 for controlling the action of the flow rate control valve 542 on thebasis of the detection output of the sensor 541. Herein, the sensor 541is composed by immersing an electrode, for example, made of platinuminto slurry. The controller 543 is designed to control the openingdegree of the flow rate control valve 542 continuously, so that the airfeed rate into the air sparger 524 may be a minimum required limit foroxidizing and digesting the sulfurous acid dissolved in the slurry fromthe combustion exhaust gas. For example, more specifically, on the basisof the correlation of the concentration of sulfurous acid andoxidation-reduction potential, the oxidation-reduction potential whenthe concentration of sulfurous acid is nearly zero is predetermined asthe reference potential, and, by proportional control, when theoxidation-reduction potential detected by the sensor 541 becomes lowerthan this reference potential, the air feed rate is increased accordingto the deviation, and when the oxidation-reduction potential detected bythe sensor 541 becomes higher than this reference potential, the airfeed rate is decreased according to the deviation.

[0362] Incidentally, since the oxidation-reduction reaction controlmeans 540 is designed to feed a minimum required limit for oxidizing thetotal volume of sulfurous acid, it eventually has a function of inducinga nearly total reduction reaction of the other acids contained in theslurry by the sulfurous acid. That is, the slight Se not removed by theelectrostatic precipitator 505 is fed into the absorption column 521together with combustion exhaust gas, and tetravalent Se (main form:selenious acid SeO₃ ²⁻) and hexavalent Se (main form: selenic acid SeO₄²⁻) exist, and the hexavalent Se is controlled by the controller 543,and reacts with the sulfurous acid absorbed from the combustion exhaustgas to undergo reduction reaction to be transformed into tetravalent Se(main form: selenious acid SeO₃ ²⁻), which takes place in the absorptioncolumn 521. This reaction is expressed in the reaction formula (11).

SeO₄ ²⁻+SO₃ ²⁻→SeO₃ ²⁻+SO₄ ²⁻  (11)

[0363] In thus constituted combustion exhaust gas treatment system, inthe upstream of the electrostatic precipitator 505, the combustionexhaust gas is sufficiently cooled, and most Se in the combustionexhaust gas is condensed and deposits on the fly ash and other dust (ofsmaller particle size in particular), and hence most of Se in thecombustion exhaust gas is captured by the electrostatic precipitator 505together with the dust. Moreover, at the upstream side of theelectrostatic precipitator 505, the liquid containing treating agent Ais sprayed from the spray pipe 561 a or 516 b, and reacts with Sedepositing on the dust in the combustion exhaust gas. Accordingly,tetravalent Se (main form: selenious acid SeO₃ ²⁻) almost completelyreacts as shown in reaction formulas (1), (2), or (3), (4), and becomesiron selenite (Fe₂(SeO₃)₃) to be insoluble, and is mixed in the removeddust.

FeCl₃→Fe³⁺+3Cl⁻  (1)

2Fe³⁺+3FeO₃ ²⁻→Fe₂(SeO₃)₃↓  (2)

[0364] or

Fe₂(SO₄)₃→2Fe³⁺+3SO₄ ²⁻  (3)

2Fe³⁺+3SeO₃ ²⁻→Fe₂(SeO₃)₃↓  (4)

[0365] In this case, the Se in the captured dust E is made insoluble bythe treating agent A, and can be directly recycled as the cementmaterial or discarded, while the Se elution standard is satisfied.

[0366] On the other hand, the combustion exhaust gas fed into theabsorption column 521 contacts with the absorbent slurry sprayed fromthe spray pipe 526 by the circulation pump 523, and sulfurous acid andSe are absorbed and removed, and the treated combustion exhaust gas isdischarged from a combustion exhaust gas lead-out unit 521 b.

[0367] The sulfurous acid absorbed in the absorbent slurry sprayed fromthe spray pipe 526 and flowing down through a filler 527 is agitated bythe air sparger 524 in the tank 522, and contacts with multiple bubblessucked in to be oxidized, and further undergoes neutralization reactionto become gypsum. In the absorption column 521, by the reaction in thereaction formula (11), nearly whole volume of hexavalent Se (main form:selenic acid SeO₄ ²⁻) is transformed into tetravalent Se (main form:selenious acid SeO₃ ²⁻). Principal reactions taking place in thisprocess (other than reaction formula (11)) are expressed in reactionformulas (15) to (17).

[0368] (Absorption Column Combustion Exhaust Gas Lead-in Part)

SO₂+H₂O→H⁺+HSO₃ ⁻  (15)

[0369] (Tank)

H⁺+HSO₃ ⁻+½O₂→2H+SO₄ ²⁻  (16)

2H⁺+SO₄ ²⁻+CaCO₃+H₂O→CaSO₄.2H₂O+CO₂  (17)

[0370] Thus, in the tank 522, gypsum (CaSO₄.2H₂O), a slight amount oflimestone (CaCO₃), and tetravalent Se (main form: selenious acid SeO₃²⁻) are suspended or dissolved, and they are sucked out by the slurrypump 531, and supplied into the solid-liquid separator 532 to beseparated into solid and liquid, and is taken out as gypsum C in a cakeform of low water content (usually water content about 10%). At thistime, if not dissolved partly, tetravalent Se (main form: selenious acidSeO₃ ²⁻) may be slightly mixed into the separated gypsum C, but themajority is sent into the separated liquid tank 533 together with theseparated liquid.

[0371] The liquid in the separated liquid tank 533 is blended with, ifnecessary, makeup water D as mentioned above, and partly sent into theabsorbent slurry tank 535 by the pump 534 to be mixed with limestone F,and is supplied again as absorbent slurry into the tank 522 tocirculate, and in this case, further, part of the liquid in theseparated liquid tank 533 is sent into the mixing means 513, and blendedwith the treating agent A, and sprayed into the combustion exhaust gas.

[0372] Accordingly, as mentioned above, the Se mainly composed oftetravalent form fed into the desulfurization apparatus 520, and otherimpurities (Cl, etc.) mixing into the circulation liquid in thedesulfurization apparatus from the combustion exhaust gas are preventedfrom being accumulated excessively in the circulation liquid system inthe desulfurization apparatus 520, and hence the high desulfurizationperformance and gypsum quality are not impaired, which eliminates therequirement of installation of wastewater treating apparatus (composedof electric dialysis machine, etc.) for the desulfurization apparatus.That is, part of the circulation liquid in the desulfurization apparatus520 is extracted sequentially as stated above, and blended with thetreating agent, then sprayed into the combustion exhaust gas, andtherefore the tetravalent Se in the circulation liquid is sequentiallymade insoluble, and is removed by the electrostatic precipitator 505 asbeing contained in the dust, together with the tetravalent Se in thecombustion exhaust gas newly introduced from the lead-in passage 515. Atthe same time, other impurities mixing in the circulation liquid aresent into the combustion exhaust gas lead-in passage 515 in the route ofmixing means 513 and pump 514, and returned to the combustion exhaustgas, and hence they are partly removed by the electrostatic precipitator505 together with the dust, or pass through the absorption column 521 tobe discharged from the combustion exhaust gas lead-out unit 521 btogether with the treated combustion exhaust gas, so that the may not beaccumulated in the circulation liquid in the desulfurization apparatus520.

[0373] As described herein, according to the combustion exhaust gastreatment system of embodiment 1, aside from the conventional combustionexhaust gas purification (removal of dust, removal of sulfurous acid),most of Se in the combustion exhaust gas can be contained in the dust inan insoluble form, so that it may be directly recycled or discarded.What is more, the hard-to-treat (make insoluble) hexavalent Se istransformed into an easy-to-discard tetravalent Se by the treating agentby the oxidation-reduction reaction control means 540 in the absorptioncolumn 521 of the desulfurization apparatus 520, and therefore, ascompared with the system comprising an independent reaction column fortransforming hexavalent Se into tetravalent Se, for example, the Se inthe combustion exhaust gas can be removed and made harmless by a simpleand inexpensive system.

[0374] According to this combustion exhaust gas treatment system,moreover, almost all of hexavalent Se is eventually transformed intotetravalent Se in the absorption column 21 by the function of theoxidation-reduction reaction control means 540, and is finally madeinsoluble and discarded, and therefore the concentration of theremaining hexavalent Se (not made insoluble) is extremely slight, andthe elution standard may be satisfied with an ample margin.

[0375] In this case, furthermore, since the circulation liquiddischarged from the desulfurization apparatus 520 is sprayed into thecombustion exhaust gas lead-in passage, without particularly requiringthe wastewater treating apparatus as mentioned above, impuritiesincluding Se are prevented from being accumulated excessively in thecirculation liquid in the desulfurization apparatus 520, and the waterin the desulfurization can be treated in a simple constitution, and thesystem can be further reduced in cost and size, while thedesulfurization performance and gypsum quality may be maintained high.

[0376] (Embodiment 2)

[0377] Embodiment 2 of the fifth invention (1) is described below. FIG.21 is a schematic structural diagram showing a constitution of acombustion exhaust gas treatment system of embodiment 2. In FIG. 21,same constituent elements as in embodiment 1 are identified with samereference numerals, and their explanations are omitted.

[0378] In the combustion exhaust gas treatment system of thisembodiment, as shown in FIG. 21, a desulfurization apparatus 560comprising a cooling and dust collecting column 561 for cooling the flueand collecting dust is disposed at the upstream side of an absorptioncolumn 521, and combustion exhaust gas containing Se and others is fedinto the cooling and dust collecting column 561 of the desulfurizationapparatus 560, and the liquid extracted from the cooling and dustcollecting column 561 is fed into mixing means 513 as dischargecirculation liquid B.

[0379] In the cooling and dust collecting column 561, the liquid in aseparated liquid tank 533 is supplied by a pump 534, and this liquid issprayed from an upper header pipe 563 by a circulation pump 562. Betweenthe cooling and dust collecting column 561 and the absorption column521, a mist eliminator, not shown, is provided.

[0380] In this constitution, impurities including Se getting into thedesulfurization apparatus 560, that is, fine dust particles not capturedby the electrostatic precipitator hardly mix into the absorption column521, but are mainly absorbed in the cooling and dust collecting column561, and are extracted as being contained in the circulation liquid B,and consequently mixed with the treating agent A and sprayed into thecombustion exhaust gas lead-in passage 515 same as in embodiment 1.

[0381] Therefore, in addition to the action and effect of embodiment 1,it is further effective to maintain a high desulfurization rate and ahigh quality (purity) of gypsum C.

[0382] (Embodiment 3)

[0383] Embodiment 3 of the fifth invention (2) is described below. FIG.22 is a schematic structural diagram showing a constitution of acombustion exhaust gas treatment system of embodiment 3. In FIG. 22,same constituent elements as in embodiment 1 are identified with samereference numerals, and their explanations are omitted.

[0384] In the combustion exhaust gas treatment system of thisembodiment, as shown in FIG. 22, without installing electrostaticprecipitator, it is characterized by feeding the combustion exhaust gascontaining fly ash and dust directly into the absorption column 521 ofthe desulfurization apparatus 520.

[0385] In this case, most of Se in the combustion exhaust gas iscontained in the dust, and enters the absorption column 521 entirely,and at least tetravalent Se thereof reacts with the sprayed treatingagent A in the combustion exhaust gas lead-in passage 515 to beinsoluble, and is separated in the solid-phase side by the solid-liquidseparator 532, and is mixed into the gypsum C in insoluble state. Ifhexavalent Se is present, it is transformed into tetravalent Se in theabsorption column 521 same as in embodiment 1, and is sequentiallyextracted as being contained in the discharge circulation liquid B, andis mixed with the treating agent A and sprayed into the combustionexhaust gas lead-in passage 515, and fed again into the desulfurizationapparatus 520, and ultimately, therefore, almost all of Se istransformed into an insoluble form as tetravalent Se and is mixed in thegypsum C.

[0386] In this case, the absorption column 521 also functions as theelectrostatic precipitator 505 in embodiment 1, and hence the facilitycost may be lower than in the system of embodiment 1.

[0387] In this embodiment, as compared with embodiment 1, the cost maybe further lowered, but due to the effects of the dust (impurities)massively mixing into the absorption column, it may be difficult torealize high desulfurization rate or high quality of gypsum C, and ifthis is a problem, it is preferred to constitute as in embodiment 1 or 2or embodiment 4 described below.

[0388] Incidentally, the dust E discharged from the heat recovery unit504 is slight, and the Se contained in the dust E is made insoluble bythe treating agent A, and hence it may be discarded directly.

[0389] (Embodiment 4)

[0390] Embodiment 4 of the fifth invention (3) is described below. FIG.23 is a schematic structural diagram showing a constitution of acombustion exhaust gas treatment system of embodiment 4. In FIG. 23,same constituent elements as in embodiment 1 are identified with samereference numerals, and their explanations are omitted.

[0391] In the combustion exhaust gas treatment system of thisembodiment, as shown in FIG. 23, a desulfurization apparatus 560comprising a cooling and dust collecting column 561 for cooling the flueand collecting dust is disposed at the upstream side of an absorptioncolumn 521, and the combustion exhaust gas not deprived of dust isdirectly fed into the cooling and dust collecting column 561 of thedesulfurization apparatus 560, and the dust slurry extracted from thecooling and dust collecting column 561 is separated into solid andliquid in the separating means 517, and the separated liquid dischargedfrom the separating means 517 is led into the mixing means 513. Theseparating means 517 is, for example, composed of a centrifugal settlingmachine, and it discharges the dust slurry led into the dust in thecooling and dust collecting column 561 as cake of low water content(dust cake G).

[0392] In this case, most of Se in combustion exhaust gas is fed intothe cooling and dust collecting column 561 as being contained in thedust, and at least tetravalent Se thereof reacts with the sprayedtreating agent A in the combustion exhaust gas lead-in passage 515 to beinsoluble, and it is separated in the solid phase side in the separatingmeans 517, and is mixed in the dust cake G in insoluble state. Ifhexavalent Se is present, it is transformed into tetravalent Se in thecooling and dust collecting column 561 same as in embodiment 2, and issequentially extracted as being contained in the discharge slurry, andis blended with the treating agent A and sprayed into the combustionexhaust gas lead-in passage 515, and is led into the desulfurizationapparatus 520 again, and finally almost all Se is made insoluble astetravalent Se, and is mixed in the dust cake G. Therefore, in thiscase, too, if the dust cake G is directly discarded, the Se elutionstandard is satisfied.

[0393] Moreover, the cooling and dust collecting column 561 alsofunction as the electrostatic precipitator 505 in embodiment 2, and itis effective to reduce the cost of equipment as compared with the systemof embodiment 2. Still more, in this embodiment, different fromembodiment 3, much dust does not mix into the absorption column 521, andhence the equipment cost may be further decreased, while highdesulfurization rate and high quality of gypsum C may be realized.

[0394] The fifth invention is not limited to the illustrated embodimentsalone, but various modifications are possible. For example, ifhexavalent Se does not exist and only other Se than hexavalent ispresent in the combustion exhaust gas in the desulfurization apparatus,the process or apparatus for reducing hexavalent Se into tetravalent Seis not needed.

[0395] The constitution of the desulfurization apparatus is not limitedto the tank oxidation type shown in the embodiments, and, for example,an oxidation column in which the slurry extracted from the absorptioncolumn is fed may be separately installed, and by blowing air into thisoxidation column, final oxidation-reduction reaction may be performedherein. In this case, too, hexavalent Se is transformed into tetravalentSe in the absorption column or oxidation column.

[0396] As the treating agent for making tetravalent Se insoluble, forexample, aside from FeCl₃, Fe₂(SO₄)₃, chelating agent (e.g. Epolus MX-7of Miyoshi Resin), or high molecular heavy metal capturing agent (e.g.Epofloc L-1 of Miyoshi Resin) may be used.

[0397] (Effects of the Fifth Invention)

[0398] According to the combustion exhaust gas treatment system of thefifth invention (1), at least the tetravalent Se in the combustionexhaust gas reacts with the treating agent sprayed into the combustionexhaust gas lead-in passage by the spraying means and becomes insolublebefore it is removed by the dust collector. Accordingly, at least thecontent of other Se than tetravalent Se is less, if the dust aftertreatment is directly recycled or discarded, the Se elution standard issatisfied.

[0399] Incidentally, if Se or other impurities may be mixed slightlyinto the circulation liquid in the desulfurization apparatus withoutbeing removed by the dust collector, most of Se becomes tetravalent, andis contained in the circulation liquid together with other impurities bythe function of the mixing means and spraying means, and extracted,blended with treating agent, and sprayed into the combustion exhaust gaslead-in passage. Accordingly, Se and impurities mixing into thecirculation liquid of the desulfurization apparatus mix into the dust tobe removed by the dust collector, and are sequentially discharged,thereby preventing excessive accumulation of these impurities into thecirculation liquid in the desulfurization apparatus. Therefore, notrequiring to install wastewater treating apparatus for thedesulfurization apparatus, the system may be reduced in cost and size,while the desulfurization performance in the desulfurization apparatusand byproduct purity may be maintained high.

[0400] According to the combustion exhaust gas treatment system of thefifth invention (2), although most Se in the combustion exhaust gas isabsorbed in the desulfurization apparatus, at least tetravalent Sethereof reacts with the treating agent sprayed into the flue lead-inpassage to be insoluble, and is discharged as being mixed in the solidmatter (gypsum, etc.) separated and formed from the slurry in thedesulfurization apparatus. The hexavalent Se is transformed intotetravalent Se by reduction reaction in the desulfurization apparatus,and is contained in the circulation liquid in the desulfurizationapparatus, and is sequentially extracted together with other impurities,and is sprayed into the combustion exhaust gas lead-in passage togetherwith the treating agent, so that it is finally discharged as being mixedin the solid matter (gypsum, etc.) separated and formed in thedesulfurization apparatus, and thereby Se and other impurities are notaccumulated in the circulation liquid.

[0401] Therefore, by this system, too, the Se elution standard may besatisfied easily, and moreover the desulfurization performance of thedesulfurization apparatus and byproduct purity can be maintained highwithout using wastewater treating apparatus.

[0402] According to the combustion exhaust gas treatment system of thefifth invention (3), although most Se in the combustion exhaust gas isabsorbed in the cooling and dust collecting column of thedesulfurization apparatus, at least tetravalent Se thereof reacts withthe treating agent sprayed by the spraying means into the flue lead-inpassage to be insoluble, and is discharged as being mixed in the solidmatter (dust cake) separated and formed by the separating means forseparating the dust slurry in the cooling and dust collecting columninto solid and liquid. The hexavalent Se is mostly transformed intotetravalent Se by reduction reaction with the sulfurous acid absorbed inthe liquid in the cooling and dust collecting column, and is containedin the filtrate in the separating means, and is sequentially extractedtogether with other impurities, and is sprayed into the combustionexhaust gas lead-in passage together with the treating agent, so that itis finally discharged as being mixed in the solid matter separated andformed by the separating means, and thereby Se and other impurities arenot accumulated in the circulation liquid.

[0403] Therefore, by this system, too, the Se elution standard may besatisfied easily, and moreover the desulfurization performance of thedesulfurization apparatus and byproduct purity can be maintained highwithout using wastewater treating apparatus. Moreover, in this system,since almost no impurity such as dust mixes into the slurry in theabsorption column in the desulfurization apparatus, the desulfurizationrate in the desulfurization apparatus, gypsum purity and otherperformances may be maintained high.

[0404] According to the combustion exhaust gas treatment system of thefifth invention (4), if hexavalent Se is present, this hexavalent Se maybe almost completely transformed into tetravalent Se in thedesulfurization apparatus, and hence the Se in the combustion exhaustgas may be treated more easily and perfectly.

1. A combustion exhaust gas treatment system for treating combustionexhaust gas containing dust and Se components, comprising means forcooling combustion exhaust gas to 350° C. or less, dust collecting meansfor separating dust in the combustion exhaust gas, and Se treating meansfor transforming the form of Se existent in the dust into an insolublecompound by adding water and treating agent to the dust separated by thedust collecting means.
 2. A combustion exhaust gas treatment system fortreating combustion exhaust gas containing dust and Se components,comprising means for cooling combustion exhaust gas to 350° C. or less,dust collecting means for separating dust in the combustion exhaust gas,Se treating means for transforming the form of Se existent in the dustinto an insoluble compound by adding water and treating agent to thedust separated by the dust collecting means, and solid-liquid separatingmeans for separating solid and liquid in the slurry, containinginsoluble Se compound, discharged from the Se treating means into solidand liquid.
 3. A combustion exhaust gas treatment system of claim 1 or2, wherein the dust collecting means comprises a plurality of recoveryunits arranged in the combustion exhaust gas duct from the inlet side tothe outlet side for separating and collecting the dust, and forintroducing only the dust collected at the outlet side into the Setreating means.
 4. A combustion exhaust gas treatment system of claim 1or 2, further comprising sorting means for sorting the dust separated bythe dust collecting means into a large particle size group and a smallparticle size group, wherein only the small particle size dust group isintroduced into the Se treating means to make Se insoluble.
 5. Acombustion exhaust gas treatment system of claim 3 or 4, furthercomprising means for mixing the dust groups, one in which Se is madeinsoluble by the Se treating means, and the remaining dust group inwhich Se is not made insoluble, so as that the moisture content is 20%or less.
 6. A combustion exhaust gas treatment system of claim 5,further comprising means for forming the dust mixed by the mixing meansinto scale.
 7. A combustion exhaust gas treatment system for removingharmful material in combustion exhaust gas, comprising means for coolingcombustion exhaust gas down to 350° C. or less, dust collecting meansfor separating dust in the combustion exhaust gas, and mixing means foradding and mixing Se elution preventive agent and humidifying liquid, orsolution of Se elution preventive agent, to the dust separated by thedust collecting means.
 8. A combustion exhaust gas treatment system forremoving harmful material in combustion exhaust gas, comprising meansfor cooling combustion exhaust gas down to 350° C. or less, dustcollecting means for separating dust in the combustion exhaust gas,mixing means for adding and mixing Se elution preventive agent andhumidifying liquid, or solution of Se elution preventive agent, to thedust separated by the dust collecting means, and means for formingscales from the dust mixed with the Se elution preventive agent andhumidifying agent, or solution of Se elution preventive agent.
 9. Acombustion exhaust gas treatment system of claim 7 or 8, wherein thedust collecting means comprises a plurality of recovery units arrangedin the combustion exhaust gas duct from the inlet side to the outletside, for separating and collecting the dust, and introducing only thedust collected at the outlet side into the mixing means.
 10. Acombustion exhaust gas treatment system of claim 7 or 8, furthercomprising sorting means for sorting the dust separated by the dustcollecting means into a large particle size group and a small particlesize group, wherein only the small particle size dust group isintroduced into the mixing means.
 11. A combustion exhaust gas treatmentsystem for treating combustion exhaust gas containing sulfurous acid,dust and Se components, comprising a dry dust collector for removingdust in the combustion exhaust gas, a desulfurization apparatus having adesulfurization column for circulating an absorbent slurry for absorbingand removing sulfurous acid, repulping means for dissolving the dustremoved by the dry dust collector in water to form slurry, treatingagent feeding means for feeding a treating agent into the dust slurryobtained in the repulping means, for transforming tetravalent Se into aninsoluble, means for separating the dust slurry into solid and liquid,and a piping system for introducing the liquid portion from theseparating means into the absorbent slurry.
 12. A combustion exhaust gastreatment system of claim 11, wherein a part of the circulating liquidin a form of the slurry in the desulfurization system is introduced intothe repulping means, and is used as solvent in the repulsing means. 13.A combustion exhaust gas treatment system of claim 11 or 12, furthercomprising means for feeding filter additive agent into the dust slurry.14. A combustion exhaust gas treatment system for treating combustionexhaust gas containing sulfurous acid, dust and Se components,comprising a dry dust collector for removing dust in the combustionexhaust gas, a desulfurization apparatus having a desulfurization columnin which an absorbent slurry for absorbing and removing sulfurous acidcirculates, means for introducing the dust removed by the dry dustcollector into the absorbent slurry, and treating agent feeding meansfor feeding a treating agent for making tetravalent Se insoluble intothe absorbent slurry.
 15. A combustion exhaust gas treatment system fortreating combustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a desulfurization apparatus having adesulfurization column in which an absorbent slurry for absorbing andremoving sulfurous acid in the combustion exhaust gas circulates, andtreating agent feeding means for feeding a treating agent into theabsorbent slurry for making tetravalent Se insoluble, wherein thecombustion exhaust gas is introduced directly into the desulfurizationcolumn.
 16. A combustion exhaust gas treatment system of any one ofclaims 11 to 15, further comprising oxidation-reduction reaction controlmeans for controlling the oxidation-reduction reaction in thedesulfurization apparatus, so that the hexavalent Se in the slurry inthe desulfurization apparatus may be reduced to tetravalent, by thesulfurous acid in the slurry.
 17. A combustion exhaust gas treatmentsystem for treating combustion exhaust gas containing sulfurous acid,dust and Se components, comprising a dry dust collector for removingdust in the combustion exhaust gas, a desulfurization apparatus having acooling and dust collecting column disposed upstream of an absorptioncolumn and a desulfurization column in which an absorbent slurry forabsorbing and removing sulfurous acid circulates, means for feeding thedust into the circulating slurry in the cooling and dust collectingcolumn, and means for feeding a treating agent into the circulatingslurry in the cooling and dust collecting column, for making tetravalentSe insoluble.
 18. A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a desulfurization apparatus having a cooling anddust collecting column disposed upstream of an absorption column and adesulfurization column in which an absorbent slurry for absorbing andremoving sulfurous acid circulates, and means for feeding a treatingagent into the circulating slurry in the cooling and dust collectingcolumn, for making tetravalent Se insoluble, wherein the combustionexhaust gas is introduced directly into the cooling and dust collectingcolumn.
 19. A combustion exhaust gas treatment system of any one ofclaims 11 to 18, further comprising a wastewater treatment apparatus fortreating wastewater from the desulfurization apparatus, and treatingagent feeding means for feeding a treating agent into the impurityslurry separated by this wastewater treatment apparatus, for makingtetravalent Se insoluble.
 20. A combustion exhaust gas treatment systemfor treating combustion exhaust gas containing dust and Se components,comprising a dust collector for removing dust from the combustionexhaust gas, and means for heating the dust removed by the dustcollector up to a temperature for gasification of Se in the dust.
 21. Acombustion exhaust gas treatment system for treating combustion exhaustgas containing sulfurous acid, dust and Se components, comprising a dustcollector for removing dust from the combustion exhaust gas, adesulfurization apparatus having an absorption column in which anabsorbent slurry for absorbing and removing sulfurous acid circulates,and means for heating the dust removed by the dust collector up to atemperature for gasification of Se in the dust, wherein the gasgenerated by heating the dust by the heating means is fed into thedesulfurization apparatus together with the combustion exhaust gas, andSe is dissolved and captured in the slurry in the desulfurizationapparatus, and a treating agent for making the tetravalent Se insolubleis mixed in the treating process of slurry.
 22. A combustion exhaust gastreatment system of claim 21, further comprising oxidation-reductionreaction control means for controlling the oxidation-reduction reactionin the desulfurization apparatus, so that the hexavalent Se in theslurry in the desulfurization apparatus may be reduced to tetravalentSe, by sulfurous acid in the slurry.
 23. A combustion exhaust gastreatment system for treating combustion exhaust gas containingsulfurous acid, dust and Se components, comprising a dust collector forremoving dust from the combustion exhaust gas, a desulfurizationapparatus having an absorption column in which an absorbent slurry forabsorbing and removing sulfurous acid circulates and a cooling and dustcollecting column disposed upstream of the absorption column, and meansfor heating the dust removed by the dust collector up to a temperaturefor gasification of Se in the dust, wherein the gas generated by heatingthe dust by the heating means is fed into the desulfurization apparatustogether with the combustion exhaust gas, and Se is dissolved andcaptured in the circulation liquid in the cooling and dust collectingcolumn, and a treating agent for making the tetravalent Se insoluble ismixed in the treating process of the circulation liquid.
 24. Acombustion exhaust gas treatment system of any one of claims 20 to 23,wherein the dust collecting means comprises a plurality of recoveryunits for separating and collecting dust from the inlet side to theoutlet side of the combustion exhaust gas, and only the dust separatedand collected from the recovery unit at the outlet side is fed into theheating means.
 25. A combustion exhaust gas treatment system of any oneof claims 20 to 23, further comprising sorting means for sorting thedust separated by the dust collecting means into a large particle sizegroup and a small particle size group, wherein only the small particlesize dust group is introduced into the heating means.
 26. A combustionexhaust gas treatment system of any one of claims 20 to 25, wherein theheating means heats the dust up to a temperature in a range of 100 to1200° C.
 27. A combustion exhaust gas treatment system for treatingcombustion exhaust gas containing sulfurous acid, dust and Secomponents, comprising a dust collector for removing dust in thecombustion exhaust gas, a desulfurization apparatus having an absorptioncolumn in which an absorbent slurry for absorbing and removing sulfurousacid circulates, means for mixing a treating agent into a circulationliquid composing the absorbent slurry extracted from the desulfurizationapparatus, for making at least tetravalent Se insoluble, and means forspraying the circulation liquid mixed with the treating agent into acombustion exhaust gas at an upstream of the dust collector.
 28. Acombustion exhaust gas treatment system for treating combustion exhaustgas containing sulfurous acid, dust and Se components, comprising adesulfurization apparatus having an absorption column in which anabsorbent slurry circulates, for absorbing and removing sulfurous acidin the combustion exhaust gas, means for mixing a treating agent into acirculation liquid composing the absorbent slurry extracted from thedesulfurization apparatus, for making at least tetravalent Se insoluble,and means for spraying the circulation liquid mixed with the treatingagent, into a combustion exhaust gas at an upstream of thedesulfurization apparatus, wherein the combustion exhaust gas isdirectly fed into the desulfurization apparatus.
 29. A combustionexhaust gas treatment system for treating combustion exhaust gascontaining sulfurous acid, dust and Se components, comprising adesulfurization apparatus having an absorption column in which anabsorbent slurry circulates, for absorbing and removing sulfurous acidin the combustion exhaust gas, and a cooling and dust collecting columndisposed upstream of the absorption column, means for separating thecirculation slurry extracted from the cooling and dust collecting columninto solid and liquid, means for mixing a treating agent into theseparated liquid discharged from the separating means, for making atleast tetravalent Se insoluble, and means for spraying the separatedliquid mixed with the treating agent into a combustion exhaust gas at anupstream of the desulfurization apparatus, wherein the combustionexhaust gas is directly fed into the cooling and dust collecting columnof the desulfurization apparatus.
 30. A combustion exhaust gas treatmentsystem of any one of claims 27 to 29, further comprisingoxidation-reduction reaction control means for controlling theoxidation-reduction reaction in the desulfurization apparatus so thatthe hexavalent Se in the slurry in the desulfurization apparatus may bereduced to tetravalent, by the sulfurous acid in the slurry.